igneous
Obsidian
Volcanic glass (SiO2-rich amorphous silicate)
Hardness: 5–6 (Mohs scale); Color: Black, gray, or dark green with brownish fluidal swirls; Luster: Vitreous/glassy; Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.4
- Hardness
- 5–6 (Mohs scale)
- Color
- Black, gray, or dark green with brownish fluidal swirls
- Luster
- Vitreous/glassy
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Physical properties
Hardness: 5–6 (Mohs scale); Color: Black, gray, or dark green with brownish fluidal swirls; Luster: Vitreous/glassy; Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.4
Formation & geological history
Formed from the rapid cooling of silica-rich (felsic) lava, which prevents crystals from growing. Usually found in volcanic regions from the Holocene to Pleistocene epochs.
Uses & applications
Used historically for sharp tools and weapons (arrowheads, blades). Today used in jewelry, ornamental carvings, and manufactured for high-end surgical scalpels.
Geological facts
Obsidian fractures with extremely sharp edges, much thinner and sharper than high-quality steel scalpels. The distinct swirls in this specimen are Flow Banding, caused by differences in composition or gas bubbles during lava flow.
Field identification & locations
Identify by its glassy texture, prominent conchoidal (shell-like) fractures, and sharp edges. Common in volcanic areas like the Western USA, Mexico, and Iceland.
