Autralian Boulder Opal

Autralian Boulder Opal

Precious opal was first found in Australia on Listowel Downs Station, south of Blackall in Western Queensland. This discovery occurred in 1868, with boulder opal mined from this deposit being sent to England for evaluation during 1873. In the same year, precious (boulder) opal, that soon became known internationally as 'Barcoo opal', was discovered on hills of the Bacroo District. Following the discovery of further boulder opal deposits in hills surrounding Kynuna, hundreds of kilometres to the north, entrepreneurs, such as Herbert Bond, began, somewhat unsuccessfully, to attempt to market Australian boulder opal to the world. But by the end of the 1870s, pioneer miner Joe Bridel had discovered a new form of precious opal at Stony Creek in the Kyabra Hills that lie north-west of Quilpie. It was the solid seams, 'pipes' and nodules of precious opal from this sandstone opal that the pioneering opal marketeer Tullie Cornthwaite Wollaston took to London in 1890 to initiate the Australian opal industry.

All Australian opal fields share the major geology in common:

• A source of silica derived by protracted chemical weathering of the sandstones of the Great Artesian Basin.
• An impermeable or semi impermeable stratum, underlying the weathered sandstone strata of the Great Artesian Basin, that traps downwards-percolating silica. This permeability barrier is provided by a claystone layer in the New South Wales, a similar strata in South Australian opal fields, and a layer of ferruginous ironstone in the Queensland boulder opal fields.
• Passageways (joints and faults) within the sandstone strata that allow the downwards flow of silica-rich solutions and their entrapment at an underlying permeability barrier. Subsequent slow evaporation of water from the trapped silica slowly allows the formation of silica spheres which flocculate into a mass of three dimensionally stacked structures of silica that eventually harden into opal.

It summary it is generally agreed that precious opal forms when large amounts of terrestrial water literally wash through the sandstone (or a similar rock) and chemically weather it. The result of this deep chemical weathering process is the production of large quantities of dissolved silica. This solution then percolates, under the influence of gravity, through joints and faults in the rock strata until it reaches an impervious level. As the silica solution comes to rest, it spreads out along this level or fills voids or suitable sights within the strata and begins to solidify. Eventually, further evaporation of water takes place and opal is formed from the trapped silica solution.

Opal is a hydrated form of amorphous silica with the chemical formula SiO₂.nH₂O.

In 1965 a group of Australian CSIRO researchers 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. 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 opal's 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.

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 6½ on Mohs' comparative scale of scratch hardness; is 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.

The value of opal depends on

• Its brightness … How bright is the overall play-of-colour?
• Its spectral range … What range of colours is visible in the play-of-colour ?
• Its saturation … How pure and vivid are the colours forming the play-of-colour
• Its pattern … What is the size, shape, regularity and rarity of the play-of-colour?
• Its consistency… Is the play-of-colour, pattern, brightness consistent or variable over the whole face of the opal?
• Its directionality … Is the play-of colour visible from all directions as the opal is rotated?

More reading is available on the webpage below:

www.gem.org.au/opal.htm