Obsidian (Igneous)
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In plain terms
Obsidian is lava that cooled so fast it didn’t have time to crystallize—so it became glass.
How it forms
Obsidian forms when silica-rich lava cools extremely quickly, preventing crystals from growing. It’s common in volcanic domes and flow margins where cooling is rapid.
What to look for
- Glassy luster; often black, but can be brown, green, or banded
- Conchoidal fracture (curved, shell-like breaks)
- Very sharp edges when freshly broken
Key takeaways
- Obsidian is volcanic glass (not crystalline).
- Conchoidal fracture is a major clue.
- Forms from very rapid cooling of silica-rich lava.
- Hydration can alter surfaces over time.
- Context (volcanic domes/flows) supports identification.