Opal Play-of-colour

The play-of-colour in precious opals may exhibit every prismatic colour from violet to red. It is produced by the diffraction of white light at the spaces between tiny, uniformly sized silica spheres that are arranged in an orderly, three-dimensional array. Depending on the diameter of the silica spheres and the angle of the incident white light one particular wavelength (prismatic colour) is reinforced by constructive interference while the others are diminished by destructive interference. The structure must consist of spheres with a uniform diameter between 200 - 350 nm to be able to produce a play-of-colour in the visible light range from violet to red (wavelength 400 - 700 nm).
Stacking faults in the silica sphere structure of opals produce many distinct colour patches and determine the Pattern of an opal's play-of-colour.

This phenomenon can be described by Bragg's law which is originally the result of experiments into the diffraction of X-rays or neutrons off crystal surfaces at certain angles:

Bragg angle adapted to the silica sphere structure of precious opal producing a play-of-colour	Scanning electron micrograph showing silica sphere structure of precious opal under 40000 x magnification

BRAGG'S LAW explains why opals with a red play-of-color are usually able to show all other prismatic colours too (at least when the stone is tilted and viewed from lower angles).

COMMON OPAL and POTCH do not exhibit a play-of-colour because the silica spheres are not uniformly sized or not arranged in an orderly pattern or because the spaces between the spheres are completely filled with silica. Sometimes it may also happen that the spheres are too small (< 200 nm) to produce a visible play-of-colour. In this case the longest wavelengths that are reinforced are part of the UV light range and therefore not visible to the human eye. If the spheres are too large (> 350 nm) there is no good play-of-colour with upright lighting conditions since the reinforced waves are located in the infrared range. However if the stone is tilted the wavelength that is reinforced becomes shorter and may move to the visible part of the spectrum so that - at first - red colours may start to appear.

PLAY-OF-COLOUR is described by the main, dominant colour followed by the accompanying colours. If the play-of-colour exhibits three or more different colours the main colour followed by the term MULTICOLOUR may be used. For example Red-multicolour stones are the rarest and most sought-after opals.

Blue only	Green with some blue	Yellow with some Green	Red with other colours
Blue	Green-Blue	Yellow-Green	Red-multicolour

Furthermore the appearance of the play-of-colour is classed by Brightness and Pattern.