The Andamooka matrix opal can be found only in the South Australia region. This particular kind of opal presents a porous host-rock. This host-rock is composed by a conglomerate of sand and small grains of precious opal. The local miners call it "kopi". Since this rock is light in colour, the colors of the opal grains, even if bright, don't appear very vivid.
However, there is a method that allows the colours of the Andamooka Matrix to stand out from the host-rock and display their wonderful brightness. The treatment of the Andamooka Matrix is done in the following way. Firstly, the rough stone is cut into sections and then soaked in a sugarish substance. Then the opal is boiled in water with the addition of a small percentage of sulphuric acid. After the just mentioned treatment, which is in no way harmful, a carbonaceous deposit will fill up the pores contained in the rock. This deposit darkens the matrix and allows the play of colours of the opal to stand out. This kind of Andamooka Matrix is very much sought after in the whole world. This is due to the difficulty of finding a gemstone with a dark background, vivid colours and an affordable price.