Posted 5 years ago
The pendant shown above is set with an Andamooka Sugar Treated Matrix Opal. This is interesting variety of treated opal from South Australia. It's a porous sandstone base (or matrix) which is shot through with fine grains of opal that fill the pores in the sandstone matrix.
This opal is 29 x 17 x 8mm, giving an estimated weight of around 20cts, making it a very impressive pendant.
When Andamooka matrix is mined it's a pale rock with very little show of colour, however it's then chemically treated to blacken the sandstone, which provides the dark base to show off the opal within.
The matrix is soaked in a saturated sugar solution and then cooked in sulphuric acid. This carbonates the sugar soaked up within the sandstone, turning it black. The process is permanent, and the stone maintains its colour forever.
Andamooka matrix can be spectacular, however it is NOT black opal, neither is it precious gem opal.
The sugar treatment process was discovered in the 1950's and for a time it was a popular way to produce attractive jewellery for the local and tourist markets. Very little is produced today, and any pieces you come across now, like the one above, are probably made in the second half of the C20th.
It's quite easy to distinguish matrix from gem opal, which has a smooth glassy surface if you look at it through a loupe, whereas matrix is a finely grained rock with minute pores.
You can just see this graininess in the pics above if you enlarge them and look at the places where the light is reflecting off the stone.