Rock IdentifierObsidian
Key takeaways
- Obsidian is volcanic glass, not a crystal — it has no crystal structure at all.
- Its razor-sharp conchoidal fracture and glassy shine are diagnostic.
- Man-made glass and black onyx are common look-alikes.
Glass from a volcano
Obsidian forms when felsic lava cools so fast that atoms never organize into crystals. The result is a natural glass — technically a mineraloid, since it lacks the ordered structure a true mineral needs. It's usually jet black, but can be brown, or show 'snowflake' patches, golden 'sheen,' or rainbow iridescence from microscopic inclusions.
How to recognize it
Obsidian is glassy and often translucent at thin edges, with a smooth, curved conchoidal fracture that leaves incredibly sharp edges — sharp enough that ancient peoples knapped it into blades. It's moderately hard (about 5–5.5) and feels warm and light compared to dense rock. Look for that combination of glassiness, sharp shell-like breaks, and lack of any crystals or grains.
Look-alikes
Manufactured glass (including 'goldstone' with its copper sparkles, and bottle glass) is the biggest confusion — man-made glass often has mold seams, perfectly uniform color, or trapped round bubbles. Black onyx and jet can also resemble it, but onyx is harder banded chalcedony and jet is a light, warm coal-like organic material. Handle broken obsidian carefully; the edges cut skin easily.
