The name of onyx indicates two completely different types of rocks: siliceous onyx normally black in color streaked with white with SiO2 composition. n H2O, similar to chalcedony (but not quartz, which is crystalline), found in Brazil, Mexico, and other places; and the calcareous onyx, also called "alabastrite onyx", "etoca onyx" or "Egyptian onyx", which is composed of CaCO3 (the one from Montaione is brown or light green that of Pakistan).
Mineralogically it is a variety of chalcedony, that is quartz in compact microcrystalline masses, of an opaque or semi-opaque, uniform color, covering red-brown tones and the entire range of grays up to black.
Like all varieties of quartz it is very hard (6 to 7 on the Mohs scale).
It is formed mainly in a low-temperature and metamorphic phylonian-hydrothermal environment, or, secondarily, in sedimentary rocks where it occurs in massive and stratified form taking the name of flint, a material widely used by man in prehistory and antiquity for the preparation of sharp objects and jewelry.