Opal Matrix is a treated opal that comes largely from a field called Andamooka in South Australia.
This particular kind of opal presents a porous host-rock composed by a conglomerate of sand filled with very small grains of precious opal. Since the sand is light in color, the colors of the opal grains, even if bright, don't appear very vivid unless it is treated.
To provide greater color contrast, opal cutters often darken the host rock by treatment with smoke or burnt sugar solutions. The result is a darkened matrix that provides a contrasting background for the opal colors to stand out.