igneous
Obsidian
Volcanic glass (SiO2-rich extrusive igneous rock)
Hardness: 5–5.5 Mohs scale. Color: Deep black, sometimes translucent at edges. Luster: Vitreous (glassy). Structure: Amorphous (non-crystalline). Cleavage: None (exhibits conchoidal fracture). Specific Gravity: 2.35–2.6.
- Hardness
- 5–5
- Color
- Deep black, sometimes translucent at edges
- Luster
- Vitreous (glassy)
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Physical properties
Hardness: 5–5.5 Mohs scale. Color: Deep black, sometimes translucent at edges. Luster: Vitreous (glassy). Structure: Amorphous (non-crystalline). Cleavage: None (exhibits conchoidal fracture). Specific Gravity: 2.35–2.6.
Formation & geological history
Formed from the rapid cooling of silica-rich (felsic) lava, which prevents the growth of mineral crystals. It is geologically young as glass eventually devitrifies into minerals; mostly found in Cenozoic sites.
Uses & applications
Used historically for sharp tools and weapons (arrowheads, knives). Currently used in jewelry, scalpel blades for delicate surgeries due to its extreme sharpness, and as decorative stone.
Geological facts
Because it lacks a crystal structure, obsidian edges can reach near-molecular thinness, making it sharper than high-quality steel scalpels. It was widely traded as a valuable commodity in ancient civilizations.
Field identification & locations
Identify by its characteristic conchoidal (curved, shell-like) fracture and sharp edges. Common in volcanic regions such as the American West (Oregon, California), Mexico, Iceland, and Italy. Note: Look for the glassy texture and absence of visible grains.
