Play of colour, body colour and transparency are the three criteria which determine the price of a fire opal. The more transparency and 'fire' it has, and the more intense the deep red of its body colour, the more valuable it will be.
The incredible play of colour is exhibited by the gemstone due to the presence of small spheres in the silica gel that causes interference and refraction appearance.
The most sought after opals are black opals, which have either dark blue, dark green, or black backgrounds and exhibit a strong play of colours. Opals known as white opals have a light tinted basic colour and also exhibit a play of colours.
This play of colour diminishes when water is lost from the stones over time and as such Opals are quite high maintenance - needing to be kept away from heat and strong light which can dry them out and cause cracks.
Its play of colours is essentially a photonic crystal phenomenon based on Bragg diffraction of light on the crystal's lattice planes. Another well-known photonic crystal is found on the wings of some butterflies such as those of genus Morpho.
The term "fire opal" is commonly used to describe the clear orange crystal opals which come from Mexico, some of which have a play of colour. Many have just an orange or reddish base with no play of colour and most of these stones are faceted.
Fire Opal: Bright orange variety of precious opal with no play of colour. Principally found in Mexico.
Flash Opal: A variety of precious opal that displays a single colour over the entire stone in a flash pattern as the stone is moved.
Black moonstone A misnomer for a dark labradorite with a bluish play of colour.
Labradorite is the plagioclase feldspar that is most commonly faceted as a gem. It comes in orange, yellow, colourless and red but the material that shows a play of colour, known as 'schiller', is the most popular for use in jewellery.
 See also: Stone, Colour, Gemstone, Jewel, Gemstones
  
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