There are two different types of Matrix Opal, coming from two different mining fields. The first type is called Boulder Matrix, which comes from the Queensland fields in the northern part of Australia’s opal regions. This natural boulder opal matrix is usually left untreated, and naturally contains a background of brown Australian ironstone with pockets of opal.
The second form of matrix opal comes from Andamooka in South Australia. This type is usually treated to give a darker background. The treatment is a soaking in sugar solution, followed by acid boiling. This causes carbon to form deposits in the tiny spaces within the rock, giving it a darker appearance similar to black opal.