obsidian, igneous rock occurring as a natural glass formed by the rapid cooling of viscous lava from volcanoes. Obsidian is extremely rich in silica (about 65 to 80 percent), is low in water, and has a chemical composition similar to rhyolite.

How and where is obsidian formed?

Obsidian is formed from quickly cooled lava, which is the parent material. Extrusive formation of obsidian may occur when felsic lava cools rapidly at the edges of a felsic lava flow or volcanic dome, or when lava cools during sudden contact with water or air.

Where is black obsidian found?

Obsidian can be found all over the world in areas with volcanic activity. For example, significant deposits of obsidian can be found in the United States, Canada, Mexico, Guatemala, Argentina, Chile, Greece, Hungary, Italy, Iceland, Russia, New Zealand, Japan, and Kenya.