Unlike ordinary opals, black opals have carbon and iron oxide trace elements present, which cause the unusual darkness of the stone. Because of their dark body tone, the rainbow colors in a black opal stand out much better than lighter opals.
This vibrancy of color makes black opal the most valuable form of all opals. Often a black opal will have a natural layer of potch (colorless opal) left on the back of the stone, which can give the stone an extra darkness and vibrancy of color. The darker this potch backing is, the darker the body tone, which usually makes for the greater vibrancy of color and a higher value black opal. This is the principle upon which doublets and triplets are based, which are an imitation of solid black opals.
Black opals are generally cut into an oval shape, however, this depends on the natural shape of the stone. Sometimes black opals are cut into freeform or teardrop shapes in order to maximize the size and carat weight of the stone. Black opals are normally cut with a low cabochon, due to the opal color bar often being much thinner in black opal than in white or crystal opals. If the color bar in an opal is thin, it is impossible to create a domed surface.