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What is opal?

FAQ :  What is opal? Where does opal form? What causes the colours in opal? What is potch? How does opal form?

Opal is one of the world's most beautiful and precious gemstones, predominantly found in Australia. It is one of only six types of precious gemstones found on planet earth, sharing prestigious company with diamonds, rubies, sapphires, emeralds, and pearls.

Over 95% of the world's precious opal comes from Australia, and opal is Australia's national gemstone.

While several different types of opal are present in nature, there are two main varieties - precious opal and common opal or potch. Precious opals exhibit the characteristic play-of-colour, and is comparatively rare. Opal which is dull and valueless is called common opal. Common opal occurs in abundance throughout the world. Common opal and potch do not exhibit a play of colour. When common opal is found in association with precious opal, it is known as potch. About 95% of all opal mined from the opal fields is common or potch, that is opal that is basically one coloured, i.e. white, grey, black, and is only suitable for backings for doublets or triplets. Of the 5% that has some colour, about 95% is only of mediocre grade - therefore only approximately 0.25 per cent has any real value at all.

Scientifically, opal is known as SiO2·nH2O - an amorphous mineraloid comprised of hydrated silicon dioxide. (A mix of silica and water). The opal's appearance can range from colourless, white, and light grey through to dark grey and black. However the truly unique thing about opal is that it displays all the colours of the spectrum in a "play of colour", resulting from the interference and diffraction of light passing through tiny silica spheres in the microstructure of opal. This means the bright rainbow colours of opal, as they appear to the human eye, will move and change dramatically depending on the angle at which the stone is viewed.

A definition of opal : "An amorphous non-crystalline gem mineral solidified from gelatinous or liquid silica deposited in cracks and cavities left by decaying vegetation, wood, crustaceans and bones. Very valuable in its 'black' forms and containing a reasonable content of water. Chemical symbol: SiO2 plus H2 O. The water content in opals can be as high as 10%. Refractive Index of 1.38 - 1.60 and a hardness of between 5.5 to 6.5 on Mohs' scale."

Precious opal is defined as opaline silica with a play of colour. The term, play of colour was created solely to describe opal's beautiful shifting of spectral hues. Australian opal is referred to as "sedimentary opal" because it is hosted predominantly by sedimentary rocks of the Mesozoic Great Artesian Basin. Australian precious opals usually contain around 5-6% water and consists of small silica spheres arranged in a regular pattern. Its hardness of 5.5 to 6.5 on Moh's scale puts it about half way down the range (roughly the same hardness as glass). It has a specific gravity of 1.9 to 2.3 depending on the amount of water present.

How is Opal Formed?   

Opal is formed from a solution of silicon dioxide and water. As water seeps through sandstone, it picks up tiny particles of silica. Millions of years ago, the solution flowed into cracks and voids in sedimentary as well as volcanic areas inland Australia. Estimates suggest this solution had a rate of deposition of approximately one centimetre thickness every five million years at a depth of forty metres. Over a period of approximately 1 to 2 million years after this period solidification occurred as the climate changed. The opal therefore remained soft and un-cemented for long periods before becoming hardened. Find out more about how opals are formed.

Why and How is Opal Coloured?   

As the silica in solution was deposited, and the water content gradually decreased, spheres formed in the gel. The spheres are formed by the particles of silica spontaneously adhering to other particles which form around it. These spheres of amorphous silica range in size from 1500 to 3500 angstroms (1 angstrom is 1 ten millionth of 1 millimetre).

The spheres are not only remarkably uniform in size but are packed, in gem quality opal, in a very regular array. Because they are spherical, there are tiny holes remaining in the structure (much the same as when marbles are placed together in a container) and these holes too are arranged in a regular three dimensional way. Therefore because of the regular array of these cavities, opal is an optical diffraction grating for visible light.   

When the spheres are bigger (about 3500 angstroms diameter) the red or orange colours are produced. And at the other end of the scale, at about 1500 angstroms diameter, the blue end of the spectrum is diffracted. Between these figures the rest of the colours of the rainbow occur.  From this it can be deduced that the light diffraction in the voids is greatest when the sphere size is greatest. Therefore red is usually the brightest colour and the blue duller. 

In summary, the colour in precious opal is caused by the regular array of silica spheres and voids diffracting white light, and breaking it into the colours of the spectrum. The diameter and spacing of the spheres controls the colour range of an opal. Small spheres produce opal of blue colour only (the most common), whereas larger spheres produce red (the rarest colour).  For more detail, see our article on how colour is formed in opals.

What is Opal's Value?   

The value of each individul opal differs greatly depending on the unique qualities of the opal. There are many determining factors, including body tone, play of colour, colours present, brilliance, pattern, and size. Please see our article on the value of opal for a more detailed explanation of how opal is valued.

Click here to download the pdf of the Opal Body Tone and Patterns Chart (courtesy of Opal Association - www.opal.asn.au)

Sources

  • "Opal in South Australia", Mines & Energy Resources, SA
  • "Opal", Qld Dept. of Mines & Energy
  • "Black Opal: A comprehensive guide to cutting and orientation", by Greg Pardey, GP Creations, 1999.
  • "Opals", by Fred Ward, Gem Book Publishers, 1997.