So what is Black Opal? Opal is non-crystalline silica, similar to quartz, but is not a mineral. Its internal structure enables unique diffraction of light to produce white, gray, red, orange, yellow, green, blue, magenta, rose, pink, slate, olive, brown, and black. Opal is formed from a solution of silica (very fine sand-like particles) and water. In some sandstone outback regions, water passes though the sandstone to form a silica-rich solution that flows to voids formed from decomposed fossils and as the water evaporates, a silica deposit is left. This is repeated over vast periods and from it, an Opal is formed.