Opals produce all their colour without the use of any pigment whatsoever. How they produce their colour is by the almost magical use of reflected and refracted light. That is they need a certain amount of light to work. No other gemstone uses refracted light to produce its colour play.
An opals chemical composition is SiO2nHO2, basically it is Hydrated Silica with a varying water content, usually between 5 and 8%. Opals have a non crystalline structure, (meaning a regular arrangement of atoms) unlike diamond, sapphire or ruby. However opal does possess a regular structure nonetheless. In precious opal, there are very organized, aligned, and concentrated chains of evenly sized tiny sphere (microns in size) all containing silicon and oxygen. Photographs of Opals taken with an electron microscope show a picture somewhat like a tray of tiny ball bearing all lined up in rows. These millions of tiny even spheres are what starts the party! Light is diffracted from these spheres at varying wavelengths. Visible light ranges from wavelengths of between 380 (Indigo)and 750 (Red) nanometers. Just think of a rainbow and how it goes from purple to red. Depending on the size of the spheres, depends on what colour is produced. The larger the spheres the longer wavelengths therefore the redder the colour show. The smaller the spheres the shorter the wavelength so producing the greens Blues and violets. The numerous ‘layers’ of spheres are the master of the play-of-colour. It is this sophisticated arrangement that has made opal famous. Producing a magical; multicoloured show, changing depending on the angles the opal is viewed from. The more uniform the size and array of the spheres the more intense the colour play. Also the way that the many arrays of spheres line up produce the many recognised patterns that you find in opal.
No two opals are the same, so if you see one that you like, if you don't buy it you will never find one like it again!!!
Now that we have explained how opal is formed and how the colours work it is obvious that for the colorplay to work properly opals need a reasonable amount of light spread across the full spectrum of visible light.
What is a full spectrum of light?
The best way to achieve the full spectrum of visible light is of course sunlight. Opals are bought sold and valued in natural sunlight. For this reason different types indoor lighting will give you different types of results. For example the typical low cost fluorescent tube lighting will in most cases kill the colorplay of everything but the best opals. This is because it produces most of its light concentrated in 530-610nm wavelength (Green to yellow) this is good for green/yellow opals. But how can a red opal produce a red colourplay if it is not getting light in that wavelength to refract? Incandescent and halogen light will give you a broader spectrum of light and therefore will give a better result. Jewelery stores will generally use metal halide, halogen or combinations of other similar types of lights as these give give off the whitest, brightest and broadest spectrum of light.
What is a reasonable amount of light?
The answer to this question is directly proportional to the quality and naturally the cost per carat of the opal. All things being equal an opal that cost $50 per carat is not going to perform in lower light the same way a $500 per carat opal is going to perform and a $5000 per carat opal will perform better again. What you will tend to find with some of the lower grade opals is that they will grey or black out in lower light and only really coming to life in direct light.
To this end and to try and make our photos as honest as possible, given the limitations of cameras and different video monitors. We take our photographs using a simple low voltage 35w halogen lamp to try and duplicate daylight situations as closely as possible.
Non precious opal or opal without colour or fire is called potch. The spheres in common opal is irregular in size and inconsistent in concentration, this, therefore produces no colour. Opals are generally single sided gems and they can range from 0 to 95% potch. What is important is the quality of the colourplay that is present. Some of the most expensive opals have the thinnest gem layer.