igneous
Black Obsidian
Volcanic Glass (SiO2)
Hardness: 5-5.5 (Mohs scale), Color: Jet black to dark gray, Luster: Vitreous/Glassy, Crystal Structure: Amorphous (non-crystalline), Cleavage: None (conchoidal fracture), Specific Gravity: 2.4
- Hardness
- 5-5
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Physical properties
Hardness: 5-5.5 (Mohs scale), Color: Jet black to dark gray, Luster: Vitreous/Glassy, Crystal Structure: Amorphous (non-crystalline), Cleavage: None (conchoidal fracture), Specific Gravity: 2.4
Formation & geological history
Formed through the rapid cooling of silica-rich (rhyolitic) lava, which prevents crystal growth. It is found in volcanic regions of various ages, typically dating from the Cenozoic era due to its tendency to devitrify over time.
Uses & applications
Used for surgical scalpel blades, ornamental jewelry, gemstones (cabochons), and historically for arrowheads and tools.
Geological facts
Obsidian fractures with extremely sharp edges—thinner than even the finest steel scalpels. It is technically a mineraloid rather than a mineral because it lacks a crystalline structure.
Field identification & locations
Identify by its glassy texture and distinctive conchoidal (shell-like) fracture patterns. Found commonly in Iceland, Mexico, and the Western United States. Collectors should look for a uniform black color and high luster.
