igneous
Black Obsidian
Amorphous volcanic glass (SiO2 + impurities)
Hardness: 5-5.5 (Mohs scale); Color: Jet black; Luster: Vitreous/glassy; Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.35-2.6.
- Hardness
- 5-5
- Color
- Jet black
- Luster
- Vitreous/glassy
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Physical properties
Hardness: 5-5.5 (Mohs scale); Color: Jet black; Luster: Vitreous/glassy; Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.35-2.6.
Formation & geological history
Formed when felsic lava from a volcanic eruption cools rapidly with minimum crystal growth. Typical in areas with geologically recent volcanic activity, dating back to various epochs depending on the flow.
Uses & applications
Commonly used for tumbled stones, decorative ornaments, gemstone jewelry, and historically for ultra-sharp surgical-grade scalpel blades and prehistoric tools.
Geological facts
Obsidian is technically a glass, not a mineral, because it lacks a crystalline structure. It was highly prized by the Aztecs and Maya for crafting weapons like the macuahuitl.
Field identification & locations
Identify in the field by its jet-black color, glassy surface, and shell-like (conchoidal) fracture patterns. Found in volcanic regions like the American West, Iceland, and Mexico. Collectors should look for a lack of internal bubbles to distinguish it from man-made glass.
