Obsidian: Formation & Identification

Q: How can you identify obsidian?

Glassy luster, smooth Almost always black (sometimes dark brown or with a greenish tint); occasionally banded Breaks with a curved, sharp-edged fracture (conchoidal fracture) — this is why it was prized historically for cutting tools and arrowheads

01 / Origin story

How does obsidian form?

Obsidian is volcanic glass — lava that cooled so fast it had no time to crystallize into minerals at all. It's technically not even made of crystals, just a solid, undifferentiated glass.

02 / Read the clues

How can you identify obsidian?

Use several clues together.
One photograph shows one specimen, not every variation.

Enlarged crop of the Obsidian specimen — Texture study · enlarged crop of this specimen

01Glassy luster, smooth
02Almost always black (sometimes dark brown or with a greenish tint); occasionally banded
03Breaks with a curved, sharp-edged fracture (conchoidal fracture) — this is why it was prized historically for cutting tools and arrowheads

03 / Commonly confused with

Similar at first glance.

Compare the materials side by side.
Photographs are not shown to the same scale.

Obsidian specimen for comparison — Obsidian ↗
Glassy luster, smooth

Basalt specimen for comparison — Basalt ↗
Fine-grained — crystals are too small to see individually, often looks almost glassy or dull

Basalt, at a glance — but obsidian's glassy shine and razor-sharp fracture edges are unmistakable once you've handled a piece; basalt is dull and doesn't fracture the same way.

04 / Out in the world