Obsidian

Obsidian is an extrusive igneous rock that occurs as a natural glass formed by rapid cooling of viscous lava from volcanoes. The color is controlled factor by trace element content and the character of inclusion.

Chemistry: SiO2, MgO and Fe2O3 ♦ Color: black ♦ Cleavage: none ♦ Fracture: conchoidal ♦ Transparency: opaque ♦ Luster: vitreous ♦ Crystal System: trigonal ♦ Moh's Hardness: 5.5 ♦ Localities: Argentina, Canada, Chile, Ecuador, Greece, Guatemala, Hungary, Iceland, Indonesia, Italy, Japan, Kenya, Mexico, New Zealand, Peru, Russia, USA.