igneous
Obsidian
Obsidian (Volcanic Glass), primarily SiO2 (Silicon Dioxide)
Hardness: 5-5.5 on Mohs scale; Color: Black to deep brown (may show sheen or rainbow effects); Luster: Vitreous (glassy); Crystal Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.35-2.6.
- Hardness
- 5-5
- Color
- Black to deep brown (may show sheen or rainbow effects)
- Luster
- Vitreous (glassy)
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Physical properties
Hardness: 5-5.5 on Mohs scale; Color: Black to deep brown (may show sheen or rainbow effects); Luster: Vitreous (glassy); Crystal Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.35-2.6.
Formation & geological history
Formed through the rapid cooling of silica-rich (felsic) lava, which prevents the growth of crystals. It is found in geologically young volcanic areas, as it is unstable over long geological time and eventually devitrifies into quartz and feldspar.
Uses & applications
Used historically for sharp tools and weapons (arrowheads, blades); modern uses include surgical scalpels for ultra-fine incisions, decorative jewelry, cabochons, and metaphysical 'protection' stones.
Geological facts
Obsidian is often called 'Nature's Glass.' Because it lacks a crystal structure, its edges can reach near-molecular thinness, making it sharper than even high-quality steel surgical scalpels.
Field identification & locations
Identify in the field by its distinctive conchoidal (shell-like) fracture patterns and sharp edges. Common locations include the Western USA (Oregon, California), Mexico, Iceland, and Japan. Collectors look for unique optical effects like gold or silver sheen and rainbow bands.
