Black opal (Black and Dark opal or semi-black) - the color of the base material is black, semi-black, dark gray to light gray. Coloring is due to the presence of carbonaceous substances in the molecular structure. It features a number of beautifully opalescent rich colors highlighted with black or dark backgrounds. It is always accompanied by a general opal, the so-called "dots" (the same color as the base mass). black opal is found in layers of clay.