igneous
Obsidian
Volcanic Glass (SiO2)
Hardness: 5-6 Mohs. Color: Black (jet black to smoky grey). Luster: Vitreous/Glassy. Structure: Amorphous (non-crystalline). Fracture: Conchoidal (shell-like curves). Specific Gravity: 2.4.
- Hardness
- 5-6 Mohs
- Color
- Black (jet black to smoky grey)
- Luster
- Vitreous/Glassy
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Physical properties
Hardness: 5-6 Mohs. Color: Black (jet black to smoky grey). Luster: Vitreous/Glassy. Structure: Amorphous (non-crystalline). Fracture: Conchoidal (shell-like curves). Specific Gravity: 2.4.
Formation & geological history
Formed by the extremely rapid cooling of silica-rich lava, preventing crystal growth. Usually associated with rhyolitic volcanic eruptions from the Cenozoic era to recent times.
Uses & applications
Historically used for cutting tools, arrowheads, and mirrors. Currently used in jewelry (cabochons), scalpel blades for delicate surgery, and as an ornamental gemstone.
Geological facts
Obsidian is naturally occurring glass. Because it has no crystal structure, its fracture edges can be thinner than a single molecule, making it sharper than high-quality steel scalpels.
Field identification & locations
Identify by its deep black color, glassy texture, and distinct curved conchoidal fractures. Commonly found in volcanic regions such as the American Northwest, Iceland, Mexico, and Japan.
