igneous
Black Obsidian
Obsidian (Volcanic Glass, SiO2 + impurities)
Hardness: 5-5.5 (Mohs scale), Color: Jet black to dark gray, Luster: Vitreous (glassy), Structure: Amorphous (non-crystalline), Cleavage: None (conchoidal fracture), Specific Gravity: 2.35-2.6.
- Hardness
- 5-5
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Physical properties
Hardness: 5-5.5 (Mohs scale), Color: Jet black to dark gray, Luster: Vitreous (glassy), Structure: Amorphous (non-crystalline), Cleavage: None (conchoidal fracture), Specific Gravity: 2.35-2.6.
Formation & geological history
Formed when silicic lava (rhyolitic) cools extremely rapidly upon extrusion from a volcano, preventing the crystallization of minerals. Most deposits are geologically young (less than few million years) as glass eventually devitrifies.
Uses & applications
Used in jewelry (beads, cabochons), ornamental carving, and in modern medicine for high-precision surgical scalpel blades because it can be sharpened to a molecular edge.
Geological facts
Unlike most rocks, obsidian is not a mineral because its structure is disordered glass rather than crystalline. It was a primary material for Stone Age toolmaking, used for arrowheads and knives.
Field identification & locations
Identified in the field by its characteristic smooth, curved conchoidal fractures (shell-like) and sharp edges. Common in volcanic areas like Iceland, the American West, and Mexico. Collectors should look for smoothness and translucency at the edges.
