Obsidian is a mineral with a hardness of 5 out of 10 on the Mohs scale of mineral hardness [?]. These Amorphously structured gems are made of silicon dioxide, their full chemical compound being SiO2.
Obsidian is an extrusive igneous rock, a volcanic glass formed by rapid cooling.
Its essential component is glass. Its accessory components are magnetite, ilmenite, oxides and other minerals, such as feldspar.
It has the same composition as dacite or rhyolite, with less than 1 percent water.
Its color is shiny black, with clear conchoidal (shell-like)fracture, its texture glassy with rare microphenocrysts and abundant opaque fine dust scattered in concentric zones.
The black color appears because of the way the embryonic crystals in the glass scatter the light. Varieties show golden or silver sheen caused by inclusions.
Obsidian may also be red-brown due to the presence of iron, and greenish or grey due to the presence of tiny gas bubbles.
It has a massive structure, with, in some cases, the presence of latent concentric fractures (perlitic fractures).
Obsidian is derived from the rapid cooling of a viscous, granitic magma. It is found as volcanic units, as fragments thrown into the air by erupting volcanos, sometimes as long filaments ("Pele's hair") and as the outer covering of rhyolitic and dacitic domes .
It is not widely distributed, although it is the commonest of natural glasses. But it is very common in recent lava flows. mostly from felsic volcanos (Japan, Java). It is found in the tertiary lavas of the Lipari Isles (Italy). The most striking occurrence is in Obsidian Cliff in Yellowstone National Park, Wyoming (USA). It is also found in Hungary and Iceland.
Obsidian is named after a Roman, "Obsius."
Shattered fragments of obsidian have sharp edges high vitreous luster, and was used in prehistoric times and antiquity for tools and sculptures.
It is currently used industrially as raw material for rock wool.
Especially prized are such varieties as snow-flake obsidian, which contains small, grey spherullites, and the silky lustred obsidian of Mexico.