What Is Obsidian? Not a Mineral at All

Obsidian isn't technically a mineral. Minerals are defined by having an ordered, repeating crystal structure, and obsidian doesn't have one — it's volcanic glass, formed when lava cools so rapidly that its atoms never get the chance to arrange themselves into that structure. It's amorphous, for essentially the same underlying reason ordinary window glass is amorphous too.

Its color and pattern come from trace mineral content and how the glass cooled: "snowflake obsidian" gets its white patches from clusters of cristobalite, a related mineral that did have time to crystallize in small pockets. Obsidian forms wherever felsic lava has cooled quickly at the surface — notable deposits include the western United States, Mexico, Iceland and Japan.