Matrix opal consists of thin seams or spots of precious opal in matrix. The matrix is usually dark ironstone, but occasionally a light sand stone. Also called Mass opal.

The term matrix opal is commonly used where the opal is intimately diffused as infillings of pores or holes between grains of the host rock in which it was formed. Boulder matrix opal is found in Queensland and can be distinguished by the ironstone host rock.

Andamooka matrix opal is a porous material from Andamooka, South Australia, which is often treated to enhance the colour by depositing black carbon by chemical treatment in the pore spaces in the stone.