Opal Colour
How Opal Colour is produced
Ok, this article on opal colour is not an earth science, geology, or chemistry lesson, but since many folks wonder what makes opals glow in a rainbow of colours, here’s a quick explanation that’s hopefully not too technical. Just in case some of the terms are unfamiliar, there’s Glossary of Terms following the text.
How Opal Colour Was Discovered 
It took the development of the electron microscope to work this out. Precious opal is made up of tiny uniform spheres of transparent hard silica, which fit together in an orderly three dimensional frame, sitting in a “bath” of silica solution. It is the orderliness of the spheres that separates precious opal from common opal.
Light passes through the transparent spheres in a direct line, but when it hits the ‘bath’ of silica, it is bent and deflected at different angles, thus producing a rainbow effect.
Deflection & Diffraction
Depending on the size of the spheres, varying colours of the spectrum are diffracted. So it is a combination of deflection (bending) and diffraction (breaking up) of light rays that creates the colour in opal. If you move the stone, light hits the spheres from different angles and bring about a change in colour. The name opal actually means “to see a change in colour.” The way in which opal colour changes within a particular stone as it is rotated and tilted is called the stone’s play of colour.
How opal colour is defined
The size of the spheres has a bearing on the colour produced. Smaller spheres bring out the blues, from one end of the spectrum. Larger spheres produce the reds from the other end. The more uniform the spheres are placed, the more intense, brilliant and defined the colour will be.
Glossary of Terms:
Amorphous
Shapeless. Not consisting of crystals. Non crystalline. Glass is amorphous. Sugar is crystalline.
Deflection
The bending of rays of light from a straight line.
The Breaking up of a ray of light into either a series of light and dark bands, or into coloured bands of the spectrum.
To spread out so as to cover a larger space or surface. To scatter.
A light produced by the electrical stimulation of a gas or vapour. Fluorescent lights have a similar effect on opal as a bright cloudy day–they do not properly bring out the colours in opal.
Hydrate
A compound produced when certain substances chemically combine with water.
Glowing with heat (red or white hot) as in a light bulb which glows white hot, but produces a light that more closely simulates natural sunlight. Sunlight and incandescent lights bring out the natural colours in opal.
Opal
Opal comes from the Latin word opalus which means to see a change in colour. Chemically, opal is hydrated silica, similar to quartz. (use graphic 1168 to show change of color in the three images)
A play of colour, similar to that of an opal.
Opaque
Not allowing light to pass through. The opposite of transparent.
Play of Colour
The way in which colours change as an opal is tilted in different directions.
S
ilica
(Silicon Dioxide) A hard, white or colourless substance, that in the form of quartz, enters into the composition of many rocks and is contained in sponges and certain plants. The needle in the mouth of a female mosquito is made of silica. Flint, sand, chalcedony, and opal are examples of silica in different forms.
The band of colours formed when a beam of white light passes through a prism or by some other means (e.g. mist or spray, in the case of a rainbow) The full range of spectrum colours are: red, orange, yellow, green blue, indigo, and violet.
A round three dimensional geometric shape whose surface is equally distant at all points from the centre point.
Letting light through without being transparent. An object that lets some light pass through
Easily seen through.(glass like) it means that you can see through it. if you take a clear glass plate and put it to your face, you can still objects on the other side.
Please leave a comment if you have any further questions about opal colour.
I recently inherited an amazing crystal collection. Among the boxes of crystal that the original owner (who passed away last year) had collected was a little bag of gems which included a couple of opals. I’ve been told they are both White Austrialian. My question is, when I hold them up to the light they loose their brilliance and just look kind of murky orange. Is that normal?
Thanks.
Sheri
Hi Sheri. Pleased to offer some comments on your opals. It sounds like they are white opals probably from the Coober Pedy field in South Australia. Opal from this area is often opaque. That means that the opal cannot be seen through and if you hold it up to the light you wont see much through it, only dull light. The other type of opal from the same area is called ‘crystal’ and this is more translucent and more valuable too. If you go through the http://www.opalmine.com encyclopedia you will find a lot of details about the different types of opal. If you like you can take a picture and send it to us for verification. There is no charge for this. Please feel free to do that and don’t hesitate to ask more questions on any of the forums in this blog. It sounds like you have a great collection there. Best wishes, Peter
Hi Peter,
Nice website, very informative.
I have an opal I bought a few years ago. I am told that it has a lot of growth lines and that this is not good.
Can you please tell me what growth lines are and why they are not good?
Many thanks,
Nila
Nila, this is a good question. Miners sometime call these growth lines ‘Kopi’ which is a clay based material which apparently mixes itself with the silica in the opal. It really looks apart of the stone but somehow destabilizes it and apparently ‘grows’ over a period of time, causing the stone to crack. I personally have not seen this happen with any stones i have owned so if someone else has some comments to make about this it would be appreciated. In the meantime, if you want to install ‘photobucket’ on your computer, you can take a picture of your stone and load it onto this online gallery. Just let me know the URL and i will take a look and make some more comments.
Nila, more to your question about growth lines. cutters them ‘potch lines’ or ‘cobwebs’ I have known in some cases of risky opal fields where these potch lines have grown and even crazed. Having said that, i owned a stone like that for around 15 years without any problems of cracking or ‘growth’ lines so it seems to be that this only occurs in opal from mines that are too damp. In actual fact another way of looking at cobwebs or growth lines is that it identifies it as a natural opal and in fact can look quite intriguing. Hope this extra information helps Nila, Peter
Tengo una mina y me interesaria vender opalo de la mejot calidad, pulido o en bruto cantera o limpio.