Tommy S. Putra  
OORA OPAL (FIRE IN STONE)
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In 1930 Sam Brooks and Roy Shepherd discovered Opal on Andamooka Pastoral lease in the far north of South Australia. Millions of years ago this area was once part of an inland sea, so consequently Opalised shells and even Opalised bones of prehistoric creatures are still being brought up to the surface. Opals are sought after precious stones and the famous Black Opal variety is especially highly prized, with prices fetching up to $30,000 per carat. Australia produces over 90%of the worlds Opals.

In the 1950's, local miners discovered how to produce an Opal product from "Matrix Opal"(a range of sandy, claylike siliceous rocks with the pore spaces filled with precious opal), by soaking the rock in sugar solution then carbonizing by gently heating in concentrated sulphuric acid. The dark carbon so produced enhanced the natural opal colours in the rock. Now a new opal product has been produced by Marijan Anic, an Opal miner and dealer.

Oora (Fire) Opal, exclusive to Krching is prepared by soaking natural opal impregnated sandstone and conglomerate in an organic solution which is then carbonized by heating to temperatures above 500 degrees Celsius. Unlike the Sulphuric Acid treatment, which only affects the surface of the rock, Oora (Fire) Opal is produced from a highly porous rock and the treatment penetrates large slabs that are later sliced or carved.

Its colour and patter displays are strong and brilliant easily compared to fine Black Opal. The material is very versatile, being suitable for jewellery, carvings watch and clock faces, mosaics, inlays for furniture picture frames etc

THE NEW GEMSTONE FROM AUSTRALIA
Oora Opal (Fire Opal) is a naturally occurring, slightly porous sandstone or sandy conglomerate in which the clay matrix and some casts have been replaced by precious opal cement. The natural body of the opal is revealed by darkening the body of the stone through the deposition of carbon in the pore spaces. A special carbonising technique dramatically enhances the natural qualities of this unique gemstone.


METHOD OF TREATMENT

A secret process of organic impregnation is followed by conversion to carbon by heating under reducing conditions to temperatures above 500oC.



DESCRIPTION

Oora Opal has a strong play of colour and the background colour of each final piece can be varied as required and as such is precious ornamental gemstone. The darker pieces have been compared with fine Black Opal.

For more information see THE AUSTRALIAN GEMMOLOGIST Volume 19 No 5, 1996, 226-232 .



SPECIFICATIONS OF OORA OPAL (type 3 treated)

APPEARANCE
Dark grey to black with 'play of colour'

HARDNESS
Ranging from Weak to very strong

SPECIFIC GRAVITY
6-7(individual clay clasts
in conglomerate may be below hardness of 5, depending on degree of silicification)

CLEAVAGE
2.0-2.2

FRACTURE
NONE

CRYSTAL SYSTEM
Conchoidal

CHEMICAL COMPOSITION
Amorphous
Hydrous silicon dioxide
SiO2,nH2O Includes sand size quartz and pebbles or
cobbles of clay stone and quartzite


SELECTED REFERENCES

Barnes, L. C., Townsend, I. J., Robertson, R. S. and Scott, D. C., 1992. Opal, South Australia's Gemstone. South Australia Department of Mines and Energy, Adelaide S A. Handbook No 5.

Brown, G., 1991. Treated Andamooka Matrix Opal, Gems and Gemology Vol 127, No. 2 pp 100-106.

Keeling, J. L., and Farrand, M. G.,1984. Origin and formation of
matrix opal from Andamooka, South Australia. Quarterly Geological Notes, Geological Survey No 90 pp3-10.



I. J. TOWNSEND BSc, FGAA
GEOLOGIST/GEMMOLOGIST
(Gemological Association of Aust)
Formerly: Office of Minerals & Energy


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May 28th 2007 23:33