Obsidian is a rock and not a mineral; it is an effusive igneous rock with full or partially glassy paste and has a homogeneous appearance. It is generated by the rapid cooling of a silicatic melt which has prevented the arrangement of silica tetrahedrons in a crystal lattice; these tetrahedrons are arranged chaotically producing an amorphous form. It owes its name to Obsidius or Obsio, the name of the man who first reported this substance in Ethiopia.
The color is always intense, dark gray, brown, greenish, bottle green, reddish black, velvety black and sometimes bluish in various shades; there are also mixed colors, streaked with brown and gray. It is opaque, translucent or transparent and has a glassy lustre. There is a variety from Utah (USA) that is used to produce ashtrays, characterized by a black color, opaque with white, spherulitic spots, often with an almost starry appearance.
It is mainly found in Italy on the Lipari island, in Mexico, in Peru, in Ethiopia. In Ethiopian obsidians, microcrystalline inclusions are common and numerous, so the structure turns out to tend to a certain order and therefore to a crystallization of the mass that includes them. Some of these obsidians rich in inclusions of very minute crystals, present the phenomenon of chatoyancy and are referred to as gating adventurine or golden obsidians. They generally come from the Caucasus and Mexico.
There are various types of obsidian with their names: Perlite, Pechstein, Marekanit, Tokayer-Luchssaphir.
Obsidian was used by ancient people to make knives and cutting weapons. The ancient Greeks prepared arrowheads with obsidian; the Romans made beautiful cups from it. These people also used it as a gem and as a mirror. The ancient Egyptians used it to carve scarabs , while the ancient people of Mexico used it to make statuettes and idols. Today obsidian is widely used as an ornamental stone in the manufacture of valuables.