Oora Opal (Aboriginal word for 'Fire')

In 1930 Sam Brooks andRoy Shepherd discovered Opal on Andamooka Pastoral lease in the farnorth of South Australia. Millions of years ago this area was once partof an inland sea, so consequently Opalised shells and even Opalisedbones of prehistoric creatures are still being brought up to thesurface. Opals are sought after precious stones and the famous BlackOpal variety is especially highly prized, with prices fetching up to$30,000 per carat. Australia produces over 90%of the worlds Opals.

Inthe 1950's, local miners discovered how to produce an Opal product from"Matrix Opal"(a range of sandy, claylike siliceous rocks with the porespaces filled with precious opal), by soaking the rock in sugarsolution then carbonizing by gently heating in concentrated sulphuricacid. The dark carbon so produced enhanced the natural opal colours inthe rock.

Oora Opal is prepared by soaking natural opalimpregnated sand stone and conglomerate in an organic solution which isthen carbonized by heating to temperatures above 500 degrees Celsius.Unlike the Sulphuric Acid treatment, which only affects the surface ofthe rock, Oora (Fire)Opal is produced from a highly porous rock and thetreatment penetrates large slabs that are later sliced or carved.

Itscolour and patter displays are strong and brilliant easily compared tofine Black Opal. The material is very versatile, being suitable forjewellery, carvings watch and clock faces, mosaics, inlays forfurniture picture frames etc

THE NEW GEMSTONE FROM AUSTRALIA

Oora Opal (Aboriginal word for'Fire') is a naturally occurring,slightly porous sandstone or sandyconglomerate in which the claymatrix and some casts have been replacedby precious opal cement. The natural body of the opal is revealed bydarkening the body of the stone through the deposition of carbon in thepore spaces. A special carbonising technique dramatically enhances thenatural qualities ofthis unique gemstone.

METHOD OF TREATMENT

A secret process of organic impregnation is followed by conversionto carbon by heating under reducing conditions to temperaturesabove500oC.

DESCRIPTION

Oora Opal has a strong play of colour and the background colour ofeach final piece can be varied as required and as such is preciousornamental gemstone. The darker pieces have been compared with fineBlack Opal.

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

Oora (Fire) Opal (type 3 treated)

APPEARANCE

Dark grey to black
with 'play of colour' ranging from

HARDNESS

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