What Is Obsidian? Not a Mineral at All

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

Why It Forms (and Why It Doesn't Always)

Obsidian specifically requires felsic lava — lava high in silica content — cooling extremely fast at the surface, typically at the edges of lava flows or domes where contact with air or water causes rapid quenching. Basaltic lava, which is lower in silica, generally crystallizes into fine-grained rock rather than glass even when cooled quickly, which is why obsidian forms in specific volcanic settings rather than wherever any lava cools fast.

Varieties

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 silica mineral) that did have time to crystallize in small pockets even as the surrounding glass didn't. Mahogany obsidian shows reddish-brown streaks from iron oxide inclusions. Rainbow and "fire" obsidian show colorful sheen from microscopic layers of gas bubbles or mineral inclusions that reflect light, similar in spirit to labradorite's layering effect though the underlying cause is different.

Long-Term Stability

Obsidian is geologically metastable — over very long timescales (typically millions of years), it can slowly devitrify, meaning it begins to crystallize internally and turn cloudy or stony rather than staying glassy. This happens far too slowly to matter for jewelry or display pieces on a human timescale.

Historical Significance

Because it fractures with an extremely sharp, predictable edge (conchoidal fracture), obsidian was one of the most important tool-making materials of the Stone Age. Its edge can be thinner than that of a surgical steel scalpel, and it's genuinely still used today in a small number of specialized surgical blades.

Where It's Found

Obsidian forms wherever felsic lava has cooled quickly at the surface — notable deposits include the western United States (Oregon, California), Mexico, Iceland and Japan.

Quick Facts

Not technically a mineral — amorphous volcanic glass — Hardness: 5–5.5 on the Mohs scale — Forms only from fast-cooled felsic lava — Historically used for the sharpest cutting edges available before metal tools.