Obsidian is a type of naturally occurring glass, produced from volcanoes (igneous rock) when a fluid felsic lava cools rapidly and freezes without sufficient time for crystal growth, for example by flowing into a body of water. It consists mainly of SiO2 (silicon dioxide), 70% or more. Obsidian is mineral-like, but not a true mineral because it is not crystalline. Its composition is very similar to that of granite and rhyolite. It is sometimes classified as a mineraloid.
The color of obsidian varies depending on the presence of impurities. Iron and magnesium typically give the obsidian a dark green to brown to black color. The inclusion of small, white, radially clustered crystals of cristobalite in the black glass produce a blotchy or snowflake pattern (snowflake obsidian). It may contain patterns of gas bubbles remaining from the lava flow, aligned along layers created as the molten rock was flowing before being cooled. These bubbles can produce interesting effects such as a golden (sheen obsidian) or rainbow sheen (rainbow obsidian). Small nuggets of obsidian that have been naturally rounded and smoothed by wind and water are called "Apache tears." Obsidian is relatively soft with a typical hardness of 5 to 5.5. Its specific gravity is approximately 2.6.