FAQ :  What are opal fossils? Where are black opal fossils found? What does opalized / opalised mean?

Lightning Ridge – Black Opal Fossils

Only one place on Earth produces black opal fossils – Lightning Ridge in northern New South Wales, Australia. Lightning Ridge is the only opal field in Australia with fossils of diverse land-living organisms – pinecones and platypuses, microscopic protozoans and gigantic dinosaurs. The fossils are usually exact replicas of plant, shell or bone material, and at times they are comprised of gem quality black opal, which is as valuable as diamonds and more beautiful.

Black opal fossils which may be found at Lightning Ridge include remnants of ancient plants, mussels, snails, crustaceans, fish, turtles, plesiosaurs, crocodiles, pterosaurs, dinosaurs, birds, and mammals. 110 million years ago the supercontinent Gondwana was a wilderness of forests of pines, ferns and palms separated by tracts of shallow sea. Dinosaurs and their relatives dominated this landscape, as well as our rare and tiny mammal ancestors. Near the edge of this ancient continent, fragments of the remains of these animals accumulated in the sands of the inland sea. Today deposits at Lightning Ridge in northern New South Wales yield some of the rarest, most beautiful and precious fossils in the world.

Lightning Ridge fossils are three-dimensional replicas of ancient organic objects, transposed into non-precious potch or precious opal. In those that are pseudomorphs, the silica has filled a simple cavity or void, like jelly in a mould, so that only the basic shape and perhaps the surface texture is preserved. However, many specimens are replacement fossils, in which intricate internal structures have been preserved by chemical alteration before the cavity was filled by the silica solution.

Most specimens at the Ridge are a combination of pseudomorph and replacement fossils. Although the transformation to silica has destroyed biomolecular evidence, marrow tissue, blood vessels, capillaries and nerve channels may be perfectly preserved. If the potch is transparent, these features are clearly visible below the surface in opalised bones. A surprising aspect is the opalisation of delicate materials like leaves and even dinosaur skin. Many pieces resemble coprolites, reptilian armour scutes or heavy scales; very occasionally, bone specimens seem to show remnants of tendons or cartilage.

This outstanding quality of preservation is partly because the opal-dirt is extremely fine-grained and an ideal casting medium. Kaolinite, smectite, and illite produce the putty-like properties of the opal clay, the smectite making it plastic and malleable.

Most opal fossils found at the Ridge consist of potch (colourless opal), therefore any fossils with colour are rare and valuable. Many fossils are damaged by machinery during excavation, as pick and shovel based operations are giving way to machine-driven excavations. Removing fossil specimens can be a delicate operation, and colourless fossil specimens are largely ignored by miners searching for colour.

South Australian Fossils

The opal fossils of the South Australian opal fields are both jewels of science and beautiful gems. The Eromanga Sea that covered the interior of Australia 100-120 million years ago was rich in marine life. Ichthyosaurs, plesiosaurs, fish, sharks, ammonites and belemnites swam in the open water. Slow-moving and sedentary animals, such as starfish, crinoids, cockles, mussels, snails and tube-worms lived on the seafloor.

Only those bones and shells that became trapped in seafloor sediment had a chance of becoming fossils. Some were replaced by clear silica, and others by precious opal.

One area which yields a vast resource of opal fossils is Moon Plain, approximately 35km from Coober Pedy in South Australia’s outback. Once part of a vast inland sea which covered most of Australia, Moon Plain once teemed with marine life, but is now a wonderland for palaeontologists. South Australia has the best cold water cretaceous marine deposits in the world, and it is difficult to imagine the extreme freezing temperatures of millions of years ago compared with today’s scorching heat.

Sources :

• “Black Opal Fossils of Lightning Ridge”, Elizabeth & Robert Smith, Kangaroo Press, 1999.
• Origin Energy Fossil Gallery
• Australian Museum Online

FAQ: Which is better, black opal or boulder opal? What is black opal? What is boulder opal? What is the difference between black opal and boulder opal? How are black opals valued? How are boulder opals valued?

Black opal has long been the most prized and most famous opal, however it’s lesser known cousin, the boulder opal, is beginning to gain recognition and popularity amongst opal lovers and the general public. So let’s compare these two types of opal and see how they stack up!

A Brief Introduction

Black opal was discovered in Australia at Lightning Ridge, New South Wales, in about the 1880’s or 1890’s and has since become famous around the world as the most brilliant and stunning opal in the world. It is highly sought after and prized around the world. Black opal is generally mined in shafts, and is usually found between 25-45 feet in ‘nobby’ (nodules of potch and colour) or ‘seam’ (horizontal deposit) formations. Black opal is considered the ‘Rolls Royce’ of opals as its dark body tone lends its colour an extra vibrancy not seen in the more common white opals found in South Australia.

Boulder opal was discovered in Australia at Quilpie, in Western Queensland, in about 1870. Since then it has risen to recognition as the second most valuable type of opal, however remains largely unknown to the general international public. Boulder opal is generally mined in open cut operations, where large amounts of dirt are cut away from the surface and removed. The ironstone boulders which contain the opal are referred to as ‘floating boulders’ due to their irregular positioning under the surface. The opal usually forms as thin veins within these boulders, and most stones are cut with the host ironstone still remaining on the back.

Body Tone

Black Opal – Black opal gets its name from its dark body tone, which ranges from dark grey to black. The darkness of the stone causes the colour to stand out much more than if the stone was white. Black opal often has a natural potch backing (colourless opal) on the back of the stone which also contributes to its dark body tone. (Note: triplets & doublets, which consist of a thin slice of opal glued to a black backing, are made to imitate black opals.)

Boulder Opal – Boulder opal also has a dark body tone (although there are some occurrences of white boulder opal), which is as dark as black opal. Because boulder opal forms in thin veins in ironstone boulders, the host ironstone is usually left on the back of the stone. This is why, if you look at the back of a boulder opal, there is a layer of brown rock attached to the back. For this reason, boulder opal is sometimes referred to as a ‘natural doublet’, as this layer of dark stone on the back gives the opal its dark body tone.

Colour & Pattern

Black Opal – Black opal may display all the brilliant colours of the rainbow, in all the patterns which opal displays, such as floral, ribbon, harlequin, straw, chinese writing, rolling flash, etc.

Boulder Opal – Boulder opal is identical to black opal in its spectrum of colours, its potential for brilliance, and in the patterns which it may display. Some argue that because boulder opal is a thin layer of opal located very close to a dark backing, it has the potential for brighter & better colour, however this is debatable.

Note: Gem quality stones (gem quality refers to extremely high quality opals) – In proportion to the amount of opal mined, there is very little difference in the quantity of rare, high quality stones which are found of each type of opal.

Stone Shape

Black Opal – The market for black opal generally demands symmetrical, oval shaped stones, with a domed cabochon, as these are the most popular shape in most jewellery.

Boulder Opal – Due to the thin nature of the opal veins which form in boulder opals, it is impossible to cut domed cabochons in most boulder opal stones. The stones are therefore usually cut into free form, irregular shapes (given the odd exception) to maximise the size of the stone and minimise the loss of opal. Boulder opals therefore often cater to a slightly different market and appeal to those who like irregular shapes in jewellery.

Strength

Black Opal – Black opal, along with white opal, is classed as having a hardness of 5.5 to 6.5 on Moh’s scale of hardness. (Diamonds being 10) Therefore it is relatively fragile stone, with a similar hardness to glass.

Boulder Opal – The opal layer in boulder opal is the same as above, however the ironstone backing which is naturally attached to the stone gives boulder opals an extra hardness and strength which gives it an advantage over other types of opal. Therefore a boulder opal will generally stand up better to impact, and be much less likely to crack than a black opal.

Rarity

Black Opal – Black opal is the rarest form of opal, and is only found in opal mining fields approximately within a 70 kilometre radius of the town of Lightning Ridge. Black opal is becoming increasingly rare and top grade black opal is not currently found anywhere else in Australia or in the world. (Unlike diamonds, which are in fact very common, black opals are genuinely rare.)

Boulder Opal – Boulder opal is currently much more readily available than black opal. The area which has the potential to yield boulder opal is much larger, and a relatively small proportion of this has been explored. The area stretches along a 200 to 300 kilometre strata in Western Queensland. Therefore, the future looks promising for boulder opal mining, as it is likely to become the only actively producing source of natural dark opal. (This is partly speculation of course).

Price

Black Opal – Because of its rarity and status, black opal carries with it a certain price attachment. Market forces determine that rare items which are highly sought-after fetch higher prices than more common items. Because of their beauty, rarity, and status, black opals fetch a much higher price in comparison to boulder opals. Some argue that black opals are overpriced, while others state simply that market forces (supply and demand) determine their value.

Boulder Opal – Boulder opals are relatively under-priced in comparison to black opals. A high quality, predominantly red black opal, which is identical to a boulder opal, may fetch prices up to seven times that of the boulder opal. Generally speaking though, boulder opals are considered to carry one third of the price of black opals, despite the fact that the cost of mining boulder opals is much more than that of black opals. Open cut mining requires much more machinery and fuel than shaft mining.

The reasons for this are as follows;

• Rarity – as stated before, boulder opals are found more commonly than black opals.
• Fame & Status – Black opals are renowned throughout the world for their rarity and beauty.
• The Ironstone Factor – Due to the natural ironstone backing and the thin nature of the opal layer, boulder opal is not traditionally priced ‘per carat’. Because ironstone is much heavier than opal, valuers consider that inclusion of the ironstone would be a distortion to a ‘per carat’ price, and therefore a lower value is applied to the stone overall. Boulder opal therefore has different valuing standards applied to it, and is sometimes valued ‘per piece’ rather than at a price per carat.  It’s important to note, however, that black opals which have a colourless black potch backing are still valued per carat, and do not receive any kind of ‘penalty’. Practically speaking, it makes little difference what is on the back of a stone once it is set into jewellery – colour and brightness are still the most important factors.

Conclusion

So there you have it. Hopefully now you’ll have an understanding of the difference between black opal and boulder opal, what makes them different, and why sometimes their value differs. It’s all part of the education process!

So which is better? The truth is – neither is better, they are both equal in quality. It all depends on the individual stone, your individual tastes, and what you value in a stone. If you value the rarity of the type of stone (the satisfaction of owning something rare), the high profile often associated with it (e.g. pink diamonds or a Rolls Royce), or an oval, dome shaped opal (as opposed to a freeshape stone), then you would probably prefer a black opal over a boulder opal. More often than not, it won’t matter what type of opal it is, once you see the one for you, you’ll fall in love with it!

• View our current stock of Australian black opals and Australian boulder opals.

A new era for opal nomenclature

Anthony Smallwood FGAA, GG
Chairman, G.A.A. Opal Nomenclature Sub-committee

Abstract

Opal is a relatively common mineral species that is found in many locations world wide. For many years a reason for the spectacular phenomenon known as play-of-colour, as seen in precious opal, remained a well hidden secret. It was not until the 1960s that Australian scientists working at the Commonwealth Scientific and Industrial Research Organisation (CSIRO) used a new instrument now known as the electron microscope to reveal the inner structure of opal and how this is responsible for generating the play of colour of precious opal.

Opal often was referred to as a ‘semi-precious gemstone’, until unique Australian black opal was discovered and successfully marketed. Today, all varieties of precious opal, which are mined in the Australian states of New South Wales, South Australia and Queensland, support a $A500 million per year industry.

One problem that the opal industry has been required to face, however, is how does one describe a gemstone that occurs both with and without a play-of-colour, in almost every colour of the rainbow, in every tone of lightness and darkness from black to white, and in every degree of transparency from opaque to perfectly transparent. Also this unique gemstone displays differences in mineralogy that reflect the varying geological environments in which it forms.

The opal nomenclature that follows is a result of three years of striving within Australia’s opal industry to achieve co-operation and to formulate a nomenclature for opal which is accepted uniformly throughout the industry.

Introduction

For many years the terminology and nomenclature used to describe opal has been widely discussed and debated by gemmologists and those members of the gem and jewellery industry who have an interest in this gemstone. Aspects of this long-running discussion can be seen in the long list of papers published throughout the forty year history of The Australian Gemmologist. But, how to best describe opal (arguably the most beautiful of gemstones) has been a contentious and difficult issue for a very long time — and may well remain so for some time to come. However, as a consequence of factors such as: growing international and local awareness of opal as a major Australian resource; the emergence world-wide of a real desire to standardise all terminology related to gemstones; and the ever growing number of synthetics and imitations that are appearing in world markets; it has became necessary to agree on some well based concepts of how a unique gem material, such as opal, should be described. It was late in 1993 that the Australian Gemstone Industry Council requested the then President of The Gemmological Association of Australia (GAA), Grahame Brown, to initiate investigations into the possibility of establishing a uniformly accepted nomenclature for opal. After a short time, a working sub-committee of the GAA was formed that consisted of representatives of The Gemmological Association of Australia, the Australian Gem Industry Association (AGIA), and the Lightning Ridge Miners Association (LRMA). Now, after three years of discussion, correspondence, and a plethora of drafted documents, and what seemed to be a never ending train of ideas and criticisms, a final draft nomenclature has been agreed-to, ratified, published, and is presented in this paper.

The Australian Gemstone Industry Council (AGIC) has accepted this nomenclature in its final draft, as has the GAA’s 1996 and 1997 Federal Conferences in Tasmania and Perth — albeit with one or two small amendments to the final draft. Now the AGIC hopes to actively progress production of a full colour publication and video on this opal nomenclature for distribution on a world-wide basis over the next twelve months. As Chairman of the GAA’s Opal Nomenclature sub-committee I would like to express my gratitude to Jack Townsend (South Australia), Kathy Endor (Queensland) and Andrew Cody (Victoria) for their untiring efforts and fruitful discussions. Also, this author wishes to express his appreciation for the work and constant liaison of the AGIA sub-committee members Glenn McKean, Drago Panich, Peter Sherman, and Peter Evans, as well as the generous support and hospitality offered by members of the LRMA — in particular Joe Schellnegger, Maxine O’Brien, and Frank Palmer.

I would encourage all members of the GAA to read and to use this nomenclature — in their every day activities, such as buying and selling, and in scientific correspondence and lectures. This nomenclature remains, according to GAA Past President Ronnie Bauer and the AGIA’s Andrew Cody, a ‘living document’. As time passes there will be, no doubt, more discussion and criticism of this nomenclature.This will be most welcome, as are any questions — all of which may be forwarded in writing to the GAA’s Opal Nomenclature Sub-committee either care of the Federal Office of the GAA at P.O. Box A791, Sydney South NSW 1235, or direct to the author at P.O. Box 692, Sutherland NSW 2232.

The nomenclature and classification of opal, that follows, is reproduced, verbatim, from the Resolutions of the Federal Council of the Gemmological Association of Australia (dated 17th May, 1997).

Opal Nomenclature and Classification

Introduction

Opal is Australia’s National Gemstone. Australia produces 95% of the world’s natural precious opal supply. This nomenclature encompasses all types and varieties of opal to provide a standardisation of terminology but does not establish any valuation methodology.

The Australian Gemstone Industry Council Inc., in collaboration with the Australian Gem Industry Association Ltd., the Gemmological Association of Australia Ltd., the Lightning Ridge Miners Association Ltd. And the Jewellers Association of Australia Ltd., has produced the following nomenclature for the classification of opal.

Opal Classification

Opal is a gemstone consisting of hydrated amorphous silica with the chemical formula SiO2.nH2O. There are two basic forms of opal described by visual appearance.

Precious Opal – is opal which exhibits the phenomenon known as play-of-colour, produced by the diffraction of white light through a micro-structure of orderly arrayed silica spheres to produce changing spectral hues.

Common Opal and Potch – is opal which does not exhibit a play-of-colour. The distinction between common opal and potch is based on formation and structure. Potch is structurally similar to precious opal but has a disorderly arrangement of silica spheres. Common opal shows some degree of micro crystallinity.

Types of Natural Opal

Natural opal is opal which has not been treated or enhanced in any way other than by cutting and polishing. There are three types of natural opal, with varieties described by the two characteristics of body tone and transparency.

Natural Opal Type 1 – is opal presented in one piece in its natural state apart from cutting or polishing and is of substantially homogenous chemical composition.

Natural Opal Type 2 – is opal presented in one piece where the opal is naturally attached to the host rock in which it was formed and the host rock is of a different chemical composition. This opal is commonly known as boulder opal.

Natural Opal Type 3 – is opal presented in one piece where the opal is intimately diffused as infillings of pores or holes or between grains of the host rock in which it was formed. This opal is commonly known as matrix opal.

Varieties of Natural Opal

The variety of natural opal is determined by the two characteristics of body tone and transparency.

Body Tone

The body tone of an opal is different to the play-of-colour displayed in precious opal. There are three varieties of natural opal based on body tone. Body tone refers to the relative darkness or lightness of the opal when ignoring the play-of-colour.

Black Opal – is the family of opal which shows a play-of-colour within or on a black body tone by reference to the AGIA Body Tone Chart N1, N2, N3 and N4 when viewed face up.

Dark Opal – is the family of opal which shows a play-of-colour within or on a dark body tone by reference to the AGIA Body Tone Chart N5, N6 when viewed face up.

Light Opal – is the family of opal which shows a play-of-colour within or on a light body tone by reference to the AGIA Body Tone chart N7, N8 or N9 when viewed face up. The N9 category is referred to as white opal.

Opal with a distinct coloured body (such as yellow, orange, red or brown) should be classified as black, dark or light opal by reference to the AGIA Body Tone Chart with a notation stating its colour hue.

Australian opal body tone chart indicator

Transparency

Opal shows all forms of diaphaneity and ranges from transparent to opaque. Natural precious opal which is transparent to semi-transparent is known as crystal opal. Crystal opal can have either a black, dark or light body colour tone. The term “crystal” in this context refers to appearance not a crystalline structure.

Opal Treatments

Opal can be subjected to various types of treatment. Present CIBJO guidelines state that any method of treatment other than standard cutting and polishing must be disclosed and the process used specified on all invoices, advertising and commercial documents. Types of treatments include colour enhancement, heating, painting, dying, resins and waxes, oiling or any application of chemicals. Opal is treated to change its natural appearance, structure or durability. Opal is colour enhanced in opal inlay jewellery where usually a thin solid crystal opal has black paint or glue applied or set above black painted jewellery.

Composite Natural Opal

Composite natural opal consists of natural opal laminates, manually cemented or attached to another material. The opal component is natural opal. There are three main forms of composite opal:

Doublet Opals – are a composition of two pieces where a slice of natural opal is cemented to a dark base material.

Triplet Opals – are a composition of three pieces where a thin slice of natural opal is cemented to a dark base material and a transparent top layer, usually of quartz or glass.

Mosaic and Chip Opals – are a composition of small flat or irregularly shaped pieces of natural opal cemented as a mosaic tile on a dark base material or encompassed in a resin.

Synthetic Opal

Synthetic Opal is material which has essentially the same chemical composition and physical structure as natural opal but has been made by laboratory or industrial process. Synthetic composites exist as synthetic doublets, triplets or mosaics and must be disclosed as synthetic composites.

Imitation Opal

Imitation Opal is material which imitates the play-of-colour of natural opal, but does not have the same physical and chemical structure or gemmological constants as natural opal.

Classification Reports

Classification reports for the following types of opal should include these details:

Natural Opal

1. Type of opal
2. Variety of opal as Black opal, Dark opal or Light opal with a body classification from N1 (Black) to N9 (White) based on the AGIA Body Tone Chart.
3. Transparency as opaque, translucent or transparent. Note if it is crystal opal.
4. Weight and dimensions

Treated Opal

1. Type of opal
2. Variety of opal as Black, Dark or Light opal
3. Transparency as opaque, translucent or transparent. Note if it is crystal opal.
4. Type of Treatment and process if known
5. Weight and dimensions

Composite Opal

1. Type of composite as doublet, triplet, mosaic or chip opal
2. Treatment process, where relevant
3. Dimensions

Synthetic and Imitation

1. Gemmological category including manufacturer (if known)
2. Description (Body Tone)
3. If composite, mention type as doublet, triplet, mosaic or chip
4. Weight and dimensions, only dimensions if composite

Origin

Any indication of the origin of opal by the use of geographical location should not be used unless it is qualified as an indication of the type of locality only as recommended by the International Confederation of Jewellery, Silverware, Diamonds, Pearls and Stones (CIBJO) such as Lightning Ridge type black opal.

How to Use the New Opal Nomenclature

This nomenclature for opal has been designed for use throughout the gemstone and jewellery industry, not only in Australia but internationally. While preparing this nomenclature, the sub-committee has been cognisant of conventions of international trade organisations, such as the International Confederation of Jewellery, Silverware, diamonds, pearls and stones (CIBJO), the International Colored Gemstone Association (ICA), as well as the linguistic problems associated with different languages and the differing connotations these languages may place on an internationally acceptable nomenclature.

This new nomenclature has not been designed to force any changes to the various colloquial terms used to describe opal in Australia, or indeed in countries overseas such as Mexico. Colourful language, Australian colloquial terms for opal, and terms that have been a part of the Australian scene for hundreds of years have added significantly to the mystique and folklore of everyday language used on the opal mining fields. Expressive local terms and older historical terms always will exist in the opal miner’s vocabulary. These will remain to have their rightful place in our gemstone history and in the tale-telling for years to come.

The purpose of the nomenclature, therefore, remains to provide a basic description of the gemstone we all prize and know as opal. This nomenclature is for everyone to use and understand. Simple descriptive terms, that can be used by the majority of people, from the customer to the scientist, have been chosen. These provide the gemstone industry as a whole with a logical and unbiased way of grading and evaluating opal. However, simple terms do become difficult when the many different types, formations, pseudomorphic fossil replacements, mineralogical types, and geological occurrences of Australian opal are considered.

Having said that, there are a few items of terminology which it is hoped this nomenclature will remove from common usage. In particular, the terms that have been deliberately removed, due to the linguistic problems they create, are ‘semi-black’, ‘grey’, and ‘solid’.

To begin with the first part of the nomenclature, mention is made of precious opal, potch and common opal. The best way of determining the difference between these is to observe whether or not the opal you are viewing shows the phenomenon which we all know as play-of-colour. It is possession of this optical phenomenon for which opal is most prized. The differentiation between these basic forms of opal is therefore quite simple. If the opal displays a play-of-colour it is termed precious opal. If a play-of-colour is not displayed, then the opal is either common or potch opal. While it is recognised that the term precious is neither a scientific nor gemmological term, it is retained in this nomenclature for simplicity, and with the intention of further enhancing the value of opal as a gemstone by removing it from any historical association with ‘semi-precious’ gemstones.

In an attempt at keeping the nomenclature simple to use, the terms common opal and potch opal have not been separated. It must be recognised, however, that there are distinct mineralogical differences between potch and common opal. (Jones & Segnit, 1971).

The term ‘solid’ has been removed from opal terminology, for the simple reason that all types of opal are essentially solid from a scientific point of view. That is, opal does not exist naturally either as a liquid or a gas. ‘Solid’ has been replaced by the gemmological term natural opal. Correlating with this use is the recommendation that when describing doublets and triplets that the term composite be used instead of ‘assembled’. This also is the terminology currently recommended by CIBJO.

Essentially there are three types or forms of natural opal, which are termed simply opal, boulder opal and matrix opal. Perhaps the most contentious issue in the nomenclature concerned introduction of the term body tone, to describe the comparative lightness or darkness of an opal as distinct from its play-of-colour. Technically, it would have been best just to have two types of ‘body tone’ — either ‘black or white’ or just ‘light or dark’. However, the sub-committee rightly decided not to attempt to change too much of the terminology that had been in common use for over a hundred years. So, inclusion of the term black opal was considered to be an imperative. Following much discussion the term body tone was included in the nomenclature to describe the comparative lightness or darkness of opal — irrespective of its play-of-colour. The term tone, which is used by colour science, is in agreement with terminology used internationally to describe the lightness or darkness of particular hues or colours.

The Scale of Body Tone ranges from N1 to N9. The prefix “N” reflects the neutral tone of this scale.The steps in the scale of body tone, which are arranged to indicate approximately equal decreases of darkness, are difficult to reproduce accurately on the printed page. A rough gauge can be obtained by printing this scale with the assistance of a good computer and a quality laser or ink jet printer.

After examining current industry standards, the N4 category was decided to be the cut-off point for black opal. The AGIA is currently attempting to produce a scale of body tone, using commercially available computer scanning devices and suitable software. However, at the time of publishing this paper, this scale is not yet available. The current reference, used by the Lightning Ridge Miners Association, is the neutral tone scale specified in the American Geological Society’s Rock-colour chart † . This has proved to be a good guide, for in most instances it will be possible to correlate the different ‘tone scales’ into a simple and repeatable system. An acceptable descriptive term was sought also to describe those opals that have distinct body colours or hues, such as those displayed by both Mexican fire opal and honey opal from Lightning Ridge — considerable amounts of which consists of common or potch opal. However, as an acceptable all round term could not be found to describe these opals, the committee decided to describe them by determining their body tone/s, their primary and secondary body colour/s or hues, and their transparency.

To determine the body tone of an opal, then, one examines the piece of opal, face-up, and determines (by visual comparison) its position in the scale of body tone.

• If the tone of the opal appears darker than N4, then the opal may be classified a black opal. Consequently, any opal with a body tone darker than N4, irrespective of hue, can correctly be termed black opal. Some boulder opal possesses this body tone, so it is very correctly termed black boulder opal. It is also appreciated that some very dark red Mexican-type opals would have dark enough body tones to be categorised as black opal.
• If the opal is lighter than N4, and its tone corresponds to N5 or N6 on the scale of body tone, then it is classified as dark opal. If, in addition, this opal has a decided hue colour, it is additionally classified as, for example, a dark blue opal.
• If, on the other hand, the tone of the opal corresponds to N7, or lighter, it is classified as light opal. If this light opal also has a hue, then it is termed, for example, a light yellow opal.

When to term an opal a crystal opal also provided considerable discussion. The key to classification as crystal opal is really the transparency of the opal. Perhaps a better term would have been ‘transparent opal’; but any change in terminology from crystal to ‘transparent’ may take many many years to progress. The obvious problem with the term crystal opal is, of course, the basic fact that that opal has no crystal structure. Again the sub-committee decided that it was unwise to change a term that had been in common use for so many years. The sub-committee further believes that overseas gemmological communities may yet force a change in this usage, if strict terminology is ever to be implemented.

The range of transparency considered acceptable for defining crystal opal (transparent to semi-transparent) was taken straight from Robert Webster’s discussion on transparency in his world-renowned textbook Gems. The committee decided that transparency did not need to be re-defined in the nomenclature; but just stated as a classifying category.

To grade the transparency of an opal with the nomenclature, how transparent the opal is must be determined. If the opal is only translucent, then it is not termed crystal opal. It should be remembered that in some instances the play-of-colour of crystal opal will be so strong or brilliant that assessment of transparency, by the normal ‘read-through’ criterion, may not be possible as the opal can not be ‘read-through’. When this occurs the best test of transparency would be to ‘look-through’ the opal with transmitted light. If transparency exists then this will be readily apparent. If the material remains only translucent, then it is correctly labelled as light opal. It is hoped that future scientific advances may yield a better and more accurate method of assessing transparency.

A note also should be made concerning the removal of the term ‘jelly’ opal. The basic facts are that due to the extreme transparency of this opal it becomes a type of lower quality crystal opal that displays subdued low quality play-of-colour. In spite of any restriction applied by this terminology the term ‘jelly’ opal will probably remain in colloquial use for many years to come.

The description of composite stones requires only a small change in nomenclature. Instead of these opals being described as ‘opal doublets’ or ‘opal triplets’, the nomenclature emphasises their composite nature by terming these doublet opals and triplet opal s. In this terminology, which emphasises the composite nature of these opals, it is the first word of the term that precisely identifies the material.

The rest of the nomenclature discusses opal treatments, synthetics and imitations. These are not associated with the descriptive nomenclature for natural opals, but have been included to complete the nomenclature. These descriptions are in accordance with the latest edition of CIBJO’s Classification of materials and Rules of application for diamonds, gemstones, and pearls.

Acknowledgements

The author wishes to thank Peter Sherman and Frank Palmer for providing specimens for examination. Rudy Weber’s photographic talents are gratefully acknowledged.

Sources : 

• Australian Opal & Gem Industry Association
• Australian Gemmologist (publication)

Safety on the opal fields

Australia produces over 90 per cent of the world’s precious opal, and more than half comes from NSW opal fields. The major opal-producing areas in NSW are Lightning Ridge and White Cliffs. Visitors may fossick or “spec” for opal on a field provided they have first obtained permission of the landowner or leaseholder. If the area is subject to a registered mineral claim, a visitor must also seek permission from the claimholder. Please see section below on “The Mining Act 1992”.

A warning

A fifteen year old boy was helping in his father’s underground opal mine when he fell six metres down an internal vertical shaft and suffered a broken leg and bruised back. He was helping his uncle and father who were mining an exploration drive about 10 metres below the surface. They had deliberately holed into the small old workings of the adjacent mine so that they could evaluate the ground ahead of them. They had one porta flood light between them that was powered by a surface generator. The electric cable to their light only stretched two metres into the old workings, but they saw what they wanted to see and the two adults returned through the opening. Unknown to the adults, the son, who was at the rear, suddenly decided to explore one of the old drives, even though he had no light to see where he was going. In the darkness, he stepped into a small unprotected six metre deep vertical internal shaft in the drive floor and fell to the bottom. He was immobilised by his injury. The rescuers gained access to where he was through an adjoining shaft.

Please exercise extreme caution whilst mining or visiting a mining field;

• Do not allow inexperienced people alone in underground mine workings
• All persons who are taken underground must be instructed and/or supervised to an appropriate level of understanding.
• Every person underground should carry a light.
• Unfamiliar workings must be considered to be dangerous and must be secured against casual entry.
• Never position a winze in the middle of a drive or cross-cut.
• School children on holidays in a work situation need constant supervision.

Fossick in Safety

Established opal fields may be good places for fossicking, but there are many hidden dangers. Children are especially vulnerable, and need to be kept under close supervision at all times. Fossickers should first make their presence known to any miners in the area. Remember heavy mining and earthmoving equipment is used on opal fields, and can operate without warning. Self-tippers and blowers bring dirt to the surface of claims from underground , often working unattended.

Shafts, drill holes and open cuts may pose a serious danger to young children. Drill holes in particular – about the width of a child’s foot – are extremely dangerous. Shafts and drill holes may be full of water and may be camouflaged by vegetation growth. Old shafts or drill holes may be in a dangerous state, with collapsed collars or protected only by rotten timber or rusted sheets of iron.

Fossickers are warned against entering old underground workings without the benefit of experienced and competent miners to assist. Many dangers exist such as unstable ground, stale air and lack of oxygen, unprotected openings underneath and holes full of water. Seek help before entering old mines. Fossicking may only take place to a depth of one metre and all holes dug must be refilled. Fossickers are also warned against entering old open cuts or costains and especially warned against fossicking into the sides of excavations which may cave in unexpectedly. Large stockpiles of dirt may also collapse unexpectedly particularly if they are undercut.

When fossicking wear strong clothing, a hat and shoes. If you are fossicking alone tell someone where you are going in case you get into difficulties. In hot weather carry plenty of water. A first aid kit is always advisable. Avoid snakes; don’t try to handle or kill them. Please note too that transport can be a problem in these areas, not only because of the distances involved, but also because of the heat. A car will be needed to visit the main fields.

Equipment for Fossicking

You will need, as a minimum:

• Pick;
• Shovels;
• Pans;
• Sieves;
• A pair of tweezers and a hand lens (x8 or x10) to help identify your find;
• Hat;
• Insect repellent;
• Boots;
• Small collecting jars.

The Mining Act 1992

Under section 12 of the Mining Act 1992, “A person must not fossick for minerals on any land that is the subject of an authority, a mineral claim or an opal prospecting licence except wih the consent of the holder of the authority, claim or licence.”

Fossickers must not use any explosives or power-operated equipment, drill or excavate to a depth of more than one metre, damage or remove any bushrock, or remove more than 20 grams of gemstones during any single period of 48 hours. A person who wishes to do more than fossicking, for example prospect and/or mine for opal, must have a licence under the Mining Act 1992. This can be an opal prospecting licence, an exploration licence, a mining lease or a mineral claim.

In traditional opal mining areas in NSW such as in Lightning Ridge and White Cliffs, prospecting for opal is mostly carried out under a mineral claim or an opal prospecting licence, if such is available. Other States may have different requirements. The maximum area allowed in NSW under a mineral claim is two hectares but within the Lightning Ridge Mineral Claims District and the White Cliffs Mineral Claims District, where special rules apply, a mineral claim is restricted to 1/4 of a hectare (i.e. 2,500 square metres).

A mineral claim authorises the holder to prospect as well as mine and a person is restricted to two claims in each District. Opal Prospecting Licences are granted over much larger areas than those granted under mineral claims but are purely for prospecting and do not authorise mining. An Opal Prospecting Licence can only be granted over lands defined as an “Opal Prospecting Block” within an area designated under the Mining Act as an “Opal Prospecting Area”. There are a number of Opal Prospecting Areas in the Lightning Ridge and White Cliffs Mineral Claims Districts.

For further information on the various titles available, the rights and duties of holders of titles and the policies that apply, contact the Department of Mineral Resources or the Mining Registrar at Lightning Ridge, or the Mining Registrar at Broken Hill.

Code of Conduct

The New South Wales opal fields are located on pastoral leases, which are still used for farming purposes. Please recognise that while visiting the fields you are a guest on a working cattle or sheep station, and respect the owner’s requirements. Do not take dogs to the fields, leave all gates as you find them, and take away your garbage. Visitors are requested to read a copy of the full code of conduct, which can be obtained from Department of Mineral Resources offices.

For more Information

Further reading:

• Opals in New South Wales, Department of Mineral Resources.
• The Mining Act 1992 (New South Wales)
• Mine Health and Safety Act 2004 (New South Wales)
• Code of conduct on the opal fields, Department of Mineral Resources.
• Gemstones in Australia, Australian Gemstone Industry Council, 1993.
• Australian precious opal: A guide book for professionals, Andrew Cody Pty Ltd, 1991.
• Australian opals and gemstones, Australian Gem Industry Association, 1987.

Places to go for information

The Department of Mineral Resources
Information Counter
29-57 Christie Street
St Leonards 2065
(PO Box 536, St Leonards NSW 1590)

Telephone: (02) 9901 8269 begin_of_the_skype_highlighting              (02) 9901 8269      end_of_the_skype_highlighting
Fax: (02) 9901 8247

OR 

Lot 60
Morilla Street
(PO Box 314)
Lightning Ridge 2834

Telephone: (02) 6829 0678 begin_of_the_skype_highlighting              (02) 6829 0678      end_of_the_skype_highlighting / 0824
Fax: (02) 6829 0825

Sources: 

• NSW Dept. of Primary Industries

The following outside resources are available for download in PDF format and require Adobe Reader.

To download a file, please right-click on the link and select ‘Save Target As’.

• OPAL Mineral Information Leaflet – QLD Govt. Dept. of Mines & Energy – October 2000.
• Opal Minfact – NSW Department of Primary Industries – January 2000.
• Strategic Assessment of the Australian Opal Industry – by David Horton (Opal Horizon) May 2001.
• Australian Sedimentary Opal – Why is Australia Unique? – by David Horton December 2002.
• What you should know about buying opal – Jewellery Association of Australia
• The origin of precious opal – by Byron Deveson
• Opal mining in Lightning ridge, review of environmental factors – NSW Dept. of Primary Industries
• Mining at Lightning Ridge – NSW Dept. of Mineral Resources

If you have seen our online opal catalogue, you will have noticed a number of categories used to describe and classify each stone. This system of offering detailed information about each stone is based on the Australian Opal Ebusiness Association’s (AOEA) Opal Classification Standard, and aims to offer as much information about each stone as possible, allowing you to make an informed decision about your opal.

Clearly there is no substitute for viewing a stone in person, however we believe that our videos and pictures (in which we take meticulous care to represent our stones accurately), combined with this classification system, allows our customers to buy with confidence. Our clients are almost always pleased with their stone once it arrives, since as a general rule a stone is better in real life than in any photograph. For a good detailed run-down on how the value of an opal is determined, please see our article on valuing opals.

Our Classifications Explained:

ID – This is the unique identification number issued to each opal when it is processed.

Category – The preface Solid means that the stone is a natural cut & polished opal which does not have any kind of backing adhered to the stone to enhance the colour (as is the case with the partially fabricated stones – doublets or triplets). Queensland Boulder Opals, even though they have a natural brown ironstone backing which makes the stone darker, are still known as solid opals since this is the natural formation of the stone. Read more about the types of stones.

Black Opal refers to opal which has a dark grey to black body tone, and is generally mined in the Lightning Ridge area of New South Wales. As a general rule, black opal is the most valuable form of opal, since its dark body tone causes the colours to be more vibrant.

Boulder Opal is opal mined in Western Queensland which normally has a natural brown ironstone backing attached to the stone. Boulder opal usually has a very dark body tone and is thus generally the second most valuable form of opal.

Crystal Opal means any kind of opal which has a translucent or transparent quality (i.e. you can see through it). Translucent or transparent stones often have an enhanced clarity of colour, and for this reason it usually increases the value of a stone. The term Crystal Opal normally denotes opal with a very light body tone, however Black Crystal Opal refers to a crystal opal which has a dark body tone.

Semi-Black Opal refers to opal which has a light to medium grey body tone and is therefore not quite dark enough to be called black opal. These opals usually fall within the ‘dark opal’ category in the Body Tone Index. (See diagram further down) Semi black opal is generally found in Lightning Ridge, but is also found in White Cliffs and occasionally South Australia. This can be one of the lesser valuable forms of opal.

White Opal means opal with a white to light body tone, and is also known as milky opal. White opal is found in large quantities in South Australia, and the bulk of it does not have the same vibrancy of colour as found in other forms of opal. For this reason, it is generally one of the least valuable forms of opals. (High quality white opal is available however.)

Setting – In the case of jewelry, this indicates the carat of the gold used, and whether it is White Gold or Yellow Gold.

Weight – This refers to the carat weight of the stone. Five carats equals approximately one gram. The value of an opal is usually determined by calculate a price ‘per carat’ according to the colour and appearance of the stone, and this value is multiplied by the carat weight. When there are multiple stones, the carat weight of all stones combined is given.

Origin – The place in Australia where the stone was mined. See our article on Australia’s opal mining fields for more detailed information on each field.

Dimensions – The dimensions (width and length of the stone facing upwards) measured in millimetres. One inch equals 25.4 millimetres. In the case of a freeshape stone, the measurements are generally given at the widest points of the stone. When there are multiple stones, the dimensions of the largest stone are given.

Thickness – The measurement in millimetres of the stone’s thickness (i.e. looking at the side of the stone). This measurement is taken as close to the centre of the stone as possible. This includes any potch (colourless opal) or ironstone which is naturally attached to the back of the stone. In the case of boulder opal, the actual layer of opal can be less than 1mm thick. Our stones are cut with enough backing on them to support and stabilise the stone and give them a good shape. We never leave extra weight or thickness on the back of a stone to boost its carat weight or value.

Body Tone Index – (See Figure 1, below). This is a device used to classify the darkness of a stone. Generally a darker stone leads to more vibrancy of colour, however it depends on the individual opal. Boulder opals are always listed as having a body tone index of 0 since they cannot be classed in the same system as other opals due to their ironstone backing. Boulder opals generally have a very dark body tone however.

Transparency – Refers to the ‘diaphaneity’ (transparency) of an opal. Opaque means the stone is not transparent. Translucent means the opal has a semi-transparent nature. Transparent means you can see through the stone. This category is used to determine whether a stone has any of the properties of a crystal opal.

Shape – Refers to the shape of the stone. Freeshape means anything which is not in a standard oval shape. Cabochon refers to the dome on the top of the stone. A cabochon can effect the appearance and pattern of a stone – for example, crystal opals often look better with high cabochon, whereas black opals can look better with a low to medium cabochon. This is up to the individual stone however and relies on the skill of the opal cutter to maximise its beauty and pattern. Low Cabochon means it has a flat or hardly any dome. Medium Cabochon means it has a medium dome. High Cabochon means it has a high dome on the surface.

Colours – Lists each colour of the spectrum which is visible in the opal. Generally the most prominent colour is listed first, then the second most prominent colour and so forth. The rarity of colours is as follows – in order of the rarest (most valuable) to the most common (least valuable). Red; Orange; Yellow; Green; Blue. Red is therefore the rarest and most highly sought-after colour in an opal, and therefore fetches the highest price. Unusual colours may also occur, such as purple and aqua which can also enhance the beauty and value of a stone. Read more about how opal displays colours in our opal colour article.

Brightness – This is one of the most important ways in which we determine the value of an opal. There are three brightness ratings – Subdued, Bright, and Brilliant. These categories are quite broad and are intended to give a general indication of a stone’s brilliance. Subdued means the stone falls into this category with the least brightness – (note that it still may be a beautiful stone). Bright means the stone has a good level of brilliance and falls into the middle category. Brilliant means the stone is a real eye-catcher – has excellent brightness and falls into the top category in this classification. Brilliant is obviously the most sought-after and valuable property for a stone as it is a very desirable quality.

Pattern – This is a description of the arrangement of the colours on the face of the stone and how they appear to the eye. The most common pattern is Floral, which we use as a very broad description meaning a random and relatively indistinct pattern of opal colours. Most opals fall into this category.

More valuable patterns include Pinfire (small dots of colour sparkling like stars), Broad Flash (large sections of colour which flash brightly at certain angles), and Rolling Flash (a large section of colour in which a bright flash rolls across a section of the stone as you move it).

Even more valuable patterns include; Ribbon (Almost indescribable – Multiple rolling flashes which line up in different sections moving next to each other and in succession), Flagstone (large distinct blocks of colour), Straw (small and thin multiple lines of colour next to each other), Chinese Writing (thin strokes of colour which look like chinese writing).

The most valuable and rare pattern is Harlequin, in which blocks of colour lie next to each other and are of approximately the same size and shape (like a checkerboard). This pattern is extremely rare and is the legendary in opal circles. Many websites on the internet use the term Harlequin very liberally, so be wary of what you are buying. Traditionally a true Harlequin opal is extremely valuable and rare, and can fetch hundreds of thousands of dollars. In two decades of experience, our opal cutter has only ever seen two true Harlequin opals, just to give you an idea of their rarity.

Notes – This is where any extra description, special characteristics, faults or interesting attributes are mentioned.

Any questions? Don’t hesitate to email us!

The Geology of Opal

FAQ :  How is opal formed? / How are opals formed? What is the Great Artesian Basin? What is potch? What causes the formation of opal?

A Simple Explanation

Opal is formed from a solution of silicon dioxide and water. As water runs down through the earth, it picks up silica from sandstone, and carries this silica-rich solution into cracks and voids , caused by natural faults or decomposing fossils. As the water evaporates, it leaves behind a silica deposit. This cycle repeats over very long periods of time, and eventually opal is formed.

Visualise the inside of an opal, like a bag of marbles. The marbles are the silica spheres.

Smaller spheres are more commonly forming, and tend to defract the purple/ blue/ green colours.

Larger spheres are rarer forming, and tend to show off the yellow/ orange/ pink and red colours. Hence why these are the rarer colours to find in Australian Opal.

A Detailed Explanation

Occasionally, when conditions are ideal, spheres of silica, contained in silica-rich solutions in the earth form and settle under gravity in a void to form layers of silica spheres. The solution is believed to have a rate of deposition of approximately one centimetre thickness in five million years at a depth of forty metres. If the process allows spheres to reach uniform size, then precious opal commences to form. For precious opal the sphere size ranges from approximately 150 to 400 nanometres producing a play of colour by diffraction in the visible light range of 400 to 700 nanometres.

Each local opal field or occurrence must have contained voids or porosity of some sort to provide a site for opal deposition. In volcanic rocks and adjacent environments the opal appears to fill only vughs and cracks whereas in sedimentary rocks there are a variety of voids created by the weathering process. Leaching of carbonate from boulders, nodules, many different fossils, along with the existing cracks, open centres of ironstone nodules and horizontal seams provide a myriad of moulds ready for the deposition of secondary minerals such as opal.

Much of the opal deposition is not precious. It is called “potch” by the miners, or common opal by the mineralogist, as it does not show a play of colour. Opaline silica not only fills the larger voids mentioned but also may fill the pore space in silt and sand size sediments cementing the grains together forming unique deposits, known as matrix, opalised sandstone or “concrete” which is a more conglomeratic unit near the base of early Cretaceous sediments.

The many variations in the types of opal depends on a number of factors. In particular, the climate provides alternating wet and dry periods, creating a rising or more importantly a falling water table which concentrates any silica in solution. The silica itself is formed either by volcanic origin or by deep weathering of Cretaceous clay sediments producing both silica and white kaolin often seen associated with the Australian opal fields. Special conditions must also prevail to slow down a falling water table in order to provide the unique situation for the production of its own variety of opal.

The chemical conditions responsible for producing opal are still being researched, however some maintain that there must be acidic conditions at some stage during the process to form silica spheres, possibly created by microbes.

While volcanic-hosted and other types of precious opal are found in Australia, virtually all economic production comes from sediment-hosted deposits associated with the Great Australian Basin. Australia has three major varieties of natural sediment-hosted precious opal – black opals from Lightning Ridge in New South Wales, white opals from South Australia, and Queensland boulder and matrix opal.

The formation of Boulder Opal

The Boulder opal found in Queensland forms in a slightly different method to other types of opal, forming inside an ironstone concretion. The concretion was formed due to ionisation, from sedimentary deposition. By definition, they are ionised concretions of varying hardness with an approximate opal composition of SiO2at 28%, Fe2O3 + AL203 at 68% and H2O at 1% composition.

The opal forms in generally elongated or ellipsoidal ironstone concretions or boulders, from a few centimetres, to up to 3 m across. The boulders may be confined to one or more layers or randomly distributed through the weathered sandstone. Their composition ranges from sandstone types (a rim or crust of ferruginised sandstone surrounding a sandstone core) or ironstone types (composed almost entirely of iron oxides).

The opal occurs as a filling or lining between the concentric layers or in radial or random cracks in the ironstone, or as a kernel in smaller concretions or nuts. (as found at Yowah and Koroit fields, the famous ‘Yowah-nuts’).

Matrix opal is where the opal occurs as a network of veins or infilling of voids or between grains of the host rock (ferruginous sandstone or ironstone). Rare seam or band opal is also found and is typically encased in ironstone.

Pipe opal occurs in pipe-like structures which may be up to several centimetres in diameter within the sandstone and these structures may be hollow or opal-filled. Wood opal is occasionally found replacing woody tissue material.

As opposed to other sedimentary precious opal, boulder opal is attached to the ironstone, and stones are usually cut with the natural ironstone backing intact. Solid opals may be cut from the ironstone material where the opal is of sufficient thickness.

Boulder opals are fashioned to standard shapes and sizes but are also cut in freeform shapes to highlight their individual beauty and to avoid wastage. Magnificent picture stones are also cut but these are mainly of interest to collectors rather than for jewellery use.

Facts about the Great Artesian Basin:

• Is one of the largest freshwater basins in the world
• Contains approximately 8,700 million megalitres of water
• Underlies 22% of Australia
• Covers a total area of 1.7 million km squared
• Supports a population of 200,000
• Underpins $3.5 billion of production annually

Sources:

“Opal in South Australia”, Mines & Energy Resources, SA
“Opal”, Qld Dept. of Mines & Energy
The Australian Gemmologist, Vol 21, #1, 2001. “Geology of Australian Opal Deposits”, L.J. Townsend.
“Lightning Ridge, Walgett & District”, information booklet, p13.
Queensland Boulder Opal Association

How to Cut and Polish Opal

FAQ:  How is opal cutting done? How do I cut rough opal? Where can I learn about cutting opals? How is opal polishing done? How can I polish an opal?

Opal cutting and polishing is a very specialised skill. Rough opal is normally purchased from the opal miners as ‘parcels’ (Bulk quantities of opal in its rough state). Potential buyers sort through the parcels and try to predict the value of stones which can be produced from the rough material.

However there is never any guarantee, as opal cutting can produce very unpredictable outcomes. Once the opal cutter has sorted through the parcel and decided which pieces are worth cutting, a diamond saw is used to cut the rough opal into ‘ rubs’ (opal in the rough shape of a stone). During this process, any excess material, cracks and potch (colourless opal) is cut off, and the piece of opal is cut into a basic stone shape.

Probably the most basic concept which any opal cutter needs is to keep the stone as large as possible, i.e. minimising waste and maximising the end size of the stone.

Each moment of cutting reduces the size of the stone, so control must be exercised.  The second basic concept is that opal can be ‘burned’ or may even crack if subjected to extreme temperatures. For this reason, water must always be used when cutting opal to avoid overheating due to friction.  ‘Burning’ a stone during polishing results in small pits forming on the surface thereby ruining the smooth surface and polish.

After the stone has been cut on the saw by hand, the opal cutter will then normally place the stones on ‘dop sticks’, consisting of nails or lengths of wood dowling, using heated wax to adhere the stone to the end of the stick. This allows a greater degree of control of the stone on the cutting wheel, especially when the stone is small. The wax is softened on a burner to permit the fixing of the stone, which is first adhered with the face of the stone pointing upwards. The face of the stone is decided by the opal cutter, considering which side has the best colour, and the best shape for the stone.

The opal cutter then uses a series of diamond grinding wheels (coarse to fine) to shape and perfect the stone. Importance is placed on removing imperfections, such as sand spots, and removing saw marks and rough spots from previous stages. The face of the stone is shaped into a cabochon (dome shape) and the shape is decided depending on the stone (normally oval). Again, maximising the size of the stone is an important consideration.

The final stage for the face is polishing. Serium Oxide is used as a polishing agent on a felt wheel with water to give the stone a beautiful polish. If the cutter is happy with the shape, and the absence of scratches, grinding marks or imperfections, he removes the stone and sticks it back on the wax with the back facing up.

The back of the stone is cut on the same set of grinding wheels, this time producing a flat bottom for the stone, and an edge which tapers up to the ‘girdle’. The shaping of the girdle is an important and difficult part of cutting, and refers to the point on the side of the stone where the two top & bottom edges meet. This edge is used by jewellers to set the stone underneath the gold, to provide a secure setting.

Boulder opal can be significantly more difficult to work. The opal forms in tiny cavities in the ironstone, therefore the seams of opal that run through the boulder can be of very excellent quality but are very thin veins from .25mm to 20 mm thick. The ironstone is generally left as backing to support the stone.

Occasionally a thick vein is deposited allowing the cutter to cut the opal in cabochon, however frequently the veins are thin and wavy, so the cutter is challenged to cut and polish the piece following the deposition of the opal, resulting in an undulating or baroque surface. Stones are generally cut into freeform shapes, which is dictated by the opal deposition and flaws within the piece.

Ironstone is also significantly harder than opal, (opal is only as hard as glass) which provides another challenge for the opal cutter. Opal will grind much quicker than ironstone, so extra care must be taken when polishing a surface comprised of both materials.

So, now you know how to cut opal! Well, not exactly… this is only a very rough guide, and it takes a lot of practice to cut opal correctly. We recommend that learners get hold of some cheaper rough material to begin with, and to get a feel for the stone. Cutting and polishing opal is a great skill, and it’s also very rewarding to uncover such beautiful colour!

Sources:

• Queensland Boulder Opal Association

FAQ:  What causes the colours in opal? How does opal get its colour? What is potch? Where do opals get their colours? What is the structure of opals? Why does opal have colour?

The colour of an opal is a magnificent thing. Unlike any other gem, opals can display all the colours of the rainbow in an iridescent, moving pattern of red, green, blue, yellow, purple, aqua, pink, and any other colour you can imagine. The pattern and arrangement of the colour which is displayed in an opal can take on many beautiful forms, and the movement of colour across the face of a stone is known as the ‘play of colour’.

This captivating miracle of nature has been admired by people the world over for centuries, and highly sought after for use in jewelry, museums and collectors’ pieces. The opal is arguably the most beautiful of all gems – at the very least it is highly unique and a true treasure of the earth. The very idea that such magnificent colours have been hidden under the earth in darkness since ancient times, and pulled out of the ground to display their opalescence in the light of day, is truly staggering.

But what causes the colors in an opal? What is different about opal that makes the colours dance and play across the face and burn in every colour of the spectrum?

The answer, put simply, is the diffraction of light. Much like a prism, which can refract white light and produce a rainbow effect, opals diffract the white light which is coming from above, displaying those amazing opal colours. To understand how this happens, it’s time for a lesson on the microscopic structure of opals;

The Structure of Opals

Basically, opal is made up of water and silica (the main component in glass). A silica solution forms when silica from under the earth mixes with water. This solution fills voids or is trapped in layers under the earth, and opal begins to form. Learn more about how opal is formed.

Over a long period of time, the solution settles and the water evaporates, allowing the gradual formation of layer upon layer of microscopic silica spheres. The spheres are formed because particles of silica spontaneously adhere to other particles which form around it. These spheres of range in size from 1500 to 3500 angstroms (1 angstrom is 1 ten millionth of 1 millimetre).

Because they are spherical, there are tiny gaps remaining between the spheres (much the same as when marbles are placed together in a container). In these gaps between the stacked spheres, a water and silica solution remains. The spheres in an opal are not only remarkably uniform in size but are packed, in gem quality opal, in a very regular array. It is these tiny spheres and gaps which hold the secret of the opal’s colour. See image, left – An electron-microscope photograph of of the ordered structure in precious opal, showing its light-diffracting spheres.

The Diffraction of Light in Opals

When white light waves enter the top of an opal, they refract and bounce around inside the opal, through all the microscopic spheres and the gaps between the spheres. As the light passes through the spheres and gaps, it diffracts (splits). Like a prism, the opal splits the white light into all the colours of the spectrum, and the light eventually bounces back out the top of the stone, at which point we get an eyeful of beautiful opal colours. The opal is the only known gemstone that is able to naturally diffract light in this way.

You may have noticed that some opals don’t have all the colours of the spectrum. Many opals can only display blue colouring, for example. This is because the diameter and spacing of the spheres controls the colour range of an opal. Getting back to our colour diffraction theory, the size and angle at which light is split determines the colour produced.

Small spheres produce opal of blue colour only (the most common), whereas larger spheres produce red (the rarest colour). When the spheres inside the opal are bigger (about 3500 angstroms diameter) the red or orange colours are produced. At the other end of the scale, at about 1500 angstroms in diameter, the blue end of the spectrum is diffracted. Between these sizes the rest of the colours of the rainbow occur.

Therefore the rarity of the colours (most common to least common) is as follows: blue, green, yellow, orange, and red. Opals which display red can also display all the other colours of the spectrum. Therefore the possible combinations of colours in an opal can be seen as: blue only, blue-green, blue-green-yellow, blue-green-yellow-orange, and finally the full spectrum of blue-green-yellow-orange-red. For this reason, the presence of red in an opal can greatly add to its value, since it is somewhat of a rarity. Opals can also contain aqua and purple as well as the other ‘non-primary’ colours which are produced when two primary colours are combined. (For example, the green and orange between the primary colours of blue, yellow, and red).

It can also be deduced that the light diffraction in the voids is greatest when the sphere size is greatest. Therefore, generally speaking, red is usually the brightest opal color and blue is duller. 

Potch, also known as common opal, is any type of opal which does not display any color. In this case, the silica spheres may be absent, too small, or too irregularly arranged to produce colour. (Opal which does display colour is known as precious opal.) Potch is virtually worthless, although it often serves as an excellent dark backing for black opals which normally have a thin segment of precious opal naturally formed on a potch backing.

FAQ : How can I tell if an opal is real? What is Gilson opal? What are synthetic opals? How can I tell if an opal is synthetic? What is synthetic opal? How can I identify opals? Is my opal real?

HOW TO TELL IF AN OPAL IS REAL

FAQ : How can I tell if an opal is real? What is Gilson opal? What are synthetic opals? How can I tell if an opal is synthetic? What is synthetic opal? How can I identify opals? Is my opal real?

SYNTHETIC OPAL, DOUBLETS & TRIPLETS

Have you ever pondered if you’re getting what you paid for? Well, hold onto your curiosity hats, because synthetic opals are a real thing. Plus, there’s a bunch of partially man-made options like triplets and doublets out there. In this article, we’re here to give you the inside scoop on how to be crystal clear about what exactly you’re bringing home. (For even more opal insights, hop over to our article on opal types where we dive deeper into the world of solids, doublets, and triplets.)

First things first, check if the stone boasts a white body tone or has that transparent charm. If it does, chances are high that it’s a genuine solid, and you’re probably looking at a splendid white or crystal opal. Now, for doublets and triplets, they’re all about that dark body tone because they rock a black artificial backing.

Sneaky Layers and Backs

Take a peek at the opal from the side. See distinct visible ‘layers’? You might be dealing with a doublet or triplet – not a full-on solid opal. In the doublet world, one of those layers is a thin slice of opal attached to a dark backing. But wait, there’s more! A triplet brings a third layer to the party, a clear, domed layer that sits right on top of the opal.

Now, turn your gaze to the back of the opal. Does it remind you of hard black or grey plastic? Triplets often cosy up to a backing of black plastic, glass, or vitrolite. Doublets, they’re a tad trickier to pin down. They might flaunt a natural potch or ironstone backing (that’s the brown boulder opal’s host rock). Here’s the real giveaway: look again at the side of the stone. Check if the ‘join’ between the opal and the backing is perfectly flat, creating a straight line around it. Most real solid opals have a little quirk in this zone – they’re curvy or bumpy, showing off their natural formation. Man-made stones? They’re all about the perfect flatness because those two sections have been meticulously glued together. But hey, if your opal’s already set in jewellery with the back and sides undercover, that’s a trickier call even for an expert.

Glassy Tops and Beyond

Is the top of the opal giving off a ‘glassy’ vibe? Triplets tend to wear a cap of hard clear plastic or quartz, giving their top a different reflective flair compared to natural opal. And here’s a tip: if you can glimpse through the opal’s top from the side, there’s a good chance it’s a triplet.

Crucial Knowledge Before Your Purchase

Before you dive into opal ownership, take a little schooling. Familiarise yourself with the look of real opal and put it side by side with your potential purchase. There’ve been tales of folks slipping coloured tinsel or foil beneath clear plastic to mimic opal’s magic. Sneaky, right?

Decoding Synthetic Solid Opals

Now, let’s talk synthetic solid opal. They’re sneaky to spot, unless you’re an expert or have clocked in plenty of experience. The pattern can be a giveaway – lab-made opals like Gilson opal rock bright colours in neat patches all over. Often, the pattern seems a tad ‘too perfect,’ almost like it’s striking a pose. Sometimes, they even flaunt a ‘snakeskin’ pattern. Not sure yet? Don’t fret. Take it over to a gemologist or an opal pro for some insights.

When Clouds Gather

Keep an eye out for cloudiness. If your opal turns cloudy after hanging around for a while, it might just be a triplet or doublet. This cloud show happens when those layers have taken a dip in water over the long haul, causing the glue to wear down and give water a ticket to the party.

Oh, and before you go, a quick note: triplets, doublets, and synthetic opals can be awesome wallet-friendly stand-ins for natural opals. Just remember to keep your gem senses sharp so you don’t find yourself overcharged or led down the garden path.

Synthetic (Gilson) Opal

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