RocksdayRocksday #4 · October 25, 2026
The answer was Obsidian
Natural volcanic glass, formed when silica-rich lava cools so fast that crystals have no time to grow.
The clues
- Type
- Igneous rock
- Color & luster
- Black, Brown, Red · Glassy
- Mohs hardness
- 5–5.5
- Texture
- Glassy
- Found in
- Glass Buttes, Oregon; Yellowstone, Wyoming
How to recognize obsidian
- Shiny, glass-like and usually black.
- Curved, shell-shaped (conchoidal) fractures with razor-sharp edges.
- Thin edges are translucent brown or gray.
- May show white 'snowflakes' or a gold/rainbow sheen.
Did you know? A fractured obsidian edge can be sharper than a steel razor blade.