Opal: Gemstone Information Opal is one of the most spectacular gemstones. A single 0pal can flash every colour of the spectrum with an intensity and quality of colour that can surpass the “fire” of the diamond. The best opal stones can command prices per carat that rival expensive diamonds, rubies and emeralds. Opal is one of the most popular gems.Fire and water opals are also very beautiful at their best. Fire opal relies on its red-orange-yellow body colour, with or without play of colour; in water opal, the play of colour appears to be suspended in a colourless transparent body. Opal with a play of colour was for years known as precious opal as distinct from common opal but the adjective is superfluous in a text dealing only with gemstones. A green translucent, rarely transparent opal, coloured by nickel, can be very attractive.Opal is a silica SiO2 with a variable amount of water, usually from 6 to 10%. Some opals are amorphous and some have a disordered crystalline structure. Opals are formed of structural units of amorphous silica and crystalline cristobalite and/or tridymite [polymorphs, with quartz, of SiO2].
Cause of the Play of Colour
Darragh and Sanders found that in opal with a play of colour the structure consists of a randomly faulted close-packed ordered array of minute transparent silica spheres which also contain a small amount of water. Additional amorphous silica cements the spheres together; in this silica, there is a slightly different water content so that there is a small difference between the RIs of the spheres and of the cementing material. Optical diffraction takes place from the array; Nassau reminds those readers with rather older textbooks that the theory of thin-film interference does not, after all, explain the play of colour.
Those familiar with opal will know that the colour shown by the different patches will change as the viewing angle alters. Only UV rays will be diffracted by very small spheres and as the size of the spheres increases, longer (visible) wavelengths will be seen until red appears.
It is easy to recognize that a sheet of small spheres of similar size will give predominantly blue to green colours and that a mixture of sphere sizes gives the best chance for a beautiful array of colours – a stone with red predominating will always cost more than another in which blue takes up most of the viewing area. Under both LW and SWUV opal may luminesce white to bluish, brownish or greenish with often persistent pale phosphorescence. The black opals are, however, generally inert, and the fire opals usually show a greenish brown response. Much common opal shows a green fluorescence, and often phosphorescence, probably from uranium.
Properties of Opal
SiO2 + 5-10% nH2O [Hydrated Silicon Dioxide]
|5.5 – 6.0
Specific Gravity (Density)
|1.90 – 2.30 (g/cm3)
Much of the finest opal occurs in thin seams and has to be recovered together with its underlying rock (matrix). The significance for fashioning is discussed below. Alternatively, some fine opal is found as nodules (‘nobbies’ in Australia). Australian opal is found chiefly in sedimentary host rocks though much precious opal, including that from Mexico, occurs with volcanic rocks.