Opal is a hydrated form of amorphous silica with the chemical formula SiO2.nH2O, where "n" varies from 1 to 21 weight per cent. Opal occurs in two forms: precious opal with a play-of-colour, and opal with no play-of-colour called potch or common opal.

Precious opal is identified and characterised by its possession of a visible play-of-colour that emanates from the opals' oft-contrasting black, dark, or light body tone. The play-of colour of precious opal consists of a random "pattern" of colour patches (or grains) that show typically "spectrally pure" colours or colours of very narrow wavelength. These colour patches often vary in size and shape or change hue (colour) as the location of any overhead light source, orientation of the opal, or direction of viewing is changed. This constitutes the pattern shown in the opal.

The cause of precious opals' play-of-colour was first discovered and described by a group of Australian CSIRO researchers in 1965. The group of eminent Australian mineralogists included Drs P.J. Darragh, A.J. Gaskin, B.J. Jones, J.V. Sanders, and E.R. Segnit. In particular, the research of Sanders, Gaskin, Jones and Segnit revealed that the play-of-colour in precious opal was caused by three dimensional close packing of its constituent transparent spheres of non-crystalline silica. These spheres have refractive indices that usually ranges from 1.435 to 1.455. As the diameter of these spheres approaches that of the wavelength of visible light, the regular three-dimensional stacking of similar sized spheres produces an array of tiny voids or holes between contacting spheres. These sphere-void interfaces act as light scattering points that form a diffraction grating at which incident white light will be bent (diffracted) into its constituent red, orange, yellow, green, blue, indigo, and violet wavelengths. So, when the diameter of the silica spheres, and hence the spacing of the voids between adjacent spheres, is of the same order as the wavelength of visible light, the spectral colours of precious opals' play-of-colour will be generated.

Simply put, as the colour or wavelength observed in the play-of-colour of precious opal is directly related to the size of its constituent spheres: blue wavelengths will be diffracted by the smaller sized spheres of ~140 nm (nanometres) diameter. Red wavelengths will be diffracted by larger sized spheres of ~240nm and as large as 300 nm diameter. Also, as a precious opal is rotated through different angles of viewing, if the spheres of silica are large enough to diffract red wavelengths of light then as the opal is rotated the full rainbow of spectral colours will be progressively observed, starting with red and progressing through orange, yellow, green, blue, indigo, and violet.

Gemmological Properties of Precious Opal

Australian precious opal from the sedimentary environment is a singly refractive non-crystalline variety of silica that has a water content usually of 3-10 per cent by weight. It has a hardness of 5 to 6.5 on Mohs' comparative scale of scratch hardness (not particularly tough) and when fractured will produce a conchoidal (shell-like) fracture surface. Australian precious opals are divided into opal (type 1), boulder opal (type 2) or matrix opal (type 3), and have body tones (relative lightness or darkness) that range from black through dark to light. The diaphaneity varies from completely opaque to completely transparent. Transparent opal that displays a play-of-colour is commonly described as crystal opal. Polished surfaces of precious opal typically display a resinous to sub-vitreous lustre. However, exceptional specimens of Australian precious opal may display a vitreous lustre.

Inclusions commonly found including Australian sedimentary precious opals include: potch (non precious opal) and potch lines, sand, gypsum, ironstone (in boulder opals), internal fractures (crazing), spider web-like superficial 'webbing' tend to follow the patterns of colour in the opal, water containing fluid inclusions, and finely dispersed minerals of various type.