Opal is a hardened gel of silica and water (usually 6 – 10%). Its physical properties vary with differences in its structure and water content. Unlike other minerals opal is amorphous, with no definite crystal structure. Opal is a common mineral, found in many countries and various geological environments. To distinguish this ordinary opal from the precious opal used in jewellery, we call it common opal.
- Chemistry: hydrated silica
- Hardness: 5.5 - 6.5
- Refractive Index: 1.44 - 1.46
- Specific gravity: 1.98 - 2.20
- Lustre: resinous to vitreous
Common opal is usually opaque white, grey or brown. Occasionally it occurs in yellow, orange, green, blue or pink. Attractively coloured, translucent material can be cut into cabochons or even faceted.
- Hyalite is a transparent, colourless opal that resembles glass
- 'Potch' is a miners' term for a grey common opal found on the Australian opal fields. It has been estimated that most of Australia's opal is potch, with precious opal comprising only a small percentage of the total.
Structure of common opal
Common opal has no play of colour. Its structure is not regular and its silica spheres vary in size. When the diameter of a silica sphere exceeds 333 nanometres (a nanometre is one millionth of a millimetre, nm), diffraction occurs only in the infrared area of the spectrum and the result is an opal with no play of colour. Such opal is usually white, grey or brown but can be coloured yellow, orange, green, blue or pink by traces of other material. Attractively coloured, translucent opal is sometimes cut into cabochons or even faceted.
When people speak of opal they generally mean precious opal. Precious opal displays a unique effect called 'play of colour', which is a pattern of spectral colours that appears to roll across the stone when it moves. Some of the more attractive patterns are given descriptive names like flame, harlequin, mosaic, pinfire, Chinese writing, ribbon and flash.
Precious opal is termed dark or light, according to its body tone (the background to its play of colour). Because dark and black opals provide the strongest contrast to brilliant colour, they are the most valuable.
Structure of Precious opal
Scanning electron microscopy has revealed that precious opal consists of a regular, three-dimensional grid of amorphous, hydrated silica spheres, uniform in size and separated by tiny voids. The differing refractive indices of spheres and voids create a three-dimensional diffraction grating, which splits white light into its spectral colours. These rays of single wavelengths overlap, reinforcing some colours and cancelling others (a process called interference). This combination of diffraction and interference produces a network of spectral colours we call play of colour.
The size of the silica spheres determines which colours are seen. The smallest sizes refract only ultraviolet radiation. When the diameter of the spheres increases to 138 nm, they can diffract visible light of the shortest wavelength, which produces violet. As the sphere diameter increases, blue, green and other colours appear. Larger spheres (over 241 nm) produce red, as well as all colours of shorter wavelength. Opals that contain red are the most colourful overall and usually the most valuable.
Small dislocations in the regular stacking of silica spheres create bounded areas of colour that change with the angle of incidence of light. Colour patterns that shift across the stone lend each opal its unique character.
To enhance their pattern of shifting colours, opals are cut and polished into a cabochon.
Other types of precious opal:
- Crystal opal is transparent to semi-transparent with play of colour throughout. Fine crystal opal comes from Mintabie, South Australia.
- Boulder opal is a very thin layer of precious opal that is indivisible from its ironstone matrix, forming a natural doublet. It is cut into shapes that follow its natural undulations. It is found only in Queensland.
- Matrix opal is Queensland boulder opal that has infiltrated small veins and patches throughout its ironstone matrix. It is polished together with the matrix to make ornamental pieces.
Australia provides about 95% of the world's precious opal. Small quantities of precious opal are found in Mexico, Brazil and the United States.
New South Wales
Lightning Ridge, New South Wales, supplies most of the world's black opal. White Cliffs, New South Wales, produces mainly light opal. Both fields have produced fine, opalised fossils, formed when opal filled spaces left by bones, shells, wood and other minerals.
Opal is also found in cavities in volcanic rocks at Tintenbar and Dubbo, New South Wales, and in thunder eggs within a rhyolite flow at Mullumbimby.
In South Australia, precious opal is mined at Andamooka, Coober Pedy and Mintabie. New mines continue to be established in both states.
Boulder opal is found over a wide area of western Queensland. This brilliantly coloured opal forms a very thin layer on an ironstone matrix and makes a natural doublet. Quilpie and Yowah are well known centres of boulder opal mining.
Watchpoint: Because opal contains water, it may crack if exposed to sudden or extreme changes in temperature or humidity.