||Psilomelane is a mixed metal oxide and in the bulk it is just a black mineral without much interest in the jewelry industry. There are a few locales in the world (one the Crown of Silver Mine, Mexico) where it can be recovered showing fine layering of the oxides which produce malachite-like patterns in a black and silver colored patterns.
Since it is made up of two or more phases, they can separate and show patterns much like malachite. This is the material that is selected for cabochon creation. Although it has a hardness of only 5 to 5.5, it works like a much harder mineral. It is not easy to dome the stone without cutting flats in to the surface. It takes a great polish but is very dirty and hard to work.