Matrix Opal from South Australia, also found sometimes in Indonesia at Banten (West Java) area. They often experience color enhancement to imitate the highly price genuine black opal.
This process is often known as "Sugar Treated" in gemology.
This processed Opal is often placed and soaked into the chemical called monosaccharides, which is still in the same group with carboxyl. This treatment uses a basic sugar acid and carbons in order to darken the color of the Opal.
After we examine the specimen with both microscope and other equipments, we see the black carbon particles inside, that are used to enhance the play of color.
Treated opals are most commonly black, although sometimes they could appear in dark blue, dark green, dark brown or grey colors.