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Where do Opals get their Colour?

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

The color 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 color 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 colors 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).

Opal under a microscope

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 colors 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.