igneous
Obsidian
Obsidian (Volcanic Glass)
Hardness: 5–5.5 Mohs scale; Color: Black to deep brown (wet look); Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: None; Fracture: Conchoidal (displays smooth, curved surfaces).
- Hardness
- 5–5
- Color
- Black to deep brown (wet look)
- Luster
- Vitreous/Glassy
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Physical properties
Hardness: 5–5.5 Mohs scale; Color: Black to deep brown (wet look); Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: None; Fracture: Conchoidal (displays smooth, curved surfaces).
Formation & geological history
Formed through the rapid cooling of silica-rich felsic lava, which prevents crystal growth. It is found in geologically recent volcanic areas (usually less than a few million years old) due to its tendency to devitrify over time.
Uses & applications
Used historically for sharp tools and weapons (arrowheads/knives); currently used for surgical scalpel blades, jewelry, decorative gemstones, and Feng Shui ornaments.
Geological facts
Obsidian breaks into edges that are thinner and sharper than a high-quality steel scalpel, even down to a single molecule thick. It has been traded across continents by ancient civilizations for over 10,000 years.
Field identification & locations
Identify by its 'shell-like' conchoidal fractures and sharp edges. In the field, look for glassy, translucent edges when held to the light. Common in volcanic regions like the American West (Iceland, Mexico, Oregon).
