Heated opal matrix vs opal

Both specimens photographed with dealer's permission at a store in Brisbane, Queensland, Australia.

Left: heated limestone matrix, Andamooka, South australia.
Right: precious opal vein on limestone matrix, Andamooka, South Australia


The limestone matrix which Andamooka opals are found with often contains small amounts of opal itself. However it is usually a pale grey or white and this is hard to see. Cooking these limestone pieces first in sugar solution and then in concentrated sulphuric acid (not recommended to do this at home!) darkens the limestone and brings out the rainbow colours. This handsome black material is sought after as a cheaper alternative to unaltered precious opal.

How to cook limestone matrix:
[link]

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