There are two different types of matrix opal, coming from two different fields. The first is called boulder matrix, which comes from the Queensland fields in the northern part of the opal regions in Australia. This natural boulder opal matrix is untreated and it naturally contains a background of brown Australian ironstone with pockets of opal in it. The other form of matrix opal comes from Andamooka in South Australia. This opal however is treated to give it a darker background. The matrix is soaked in sugar solution and boiled in acid. This causes carbon to deposit in the spaces in the rock, giving it a dark or black opal appearance.