igneous
Obsidian
Volcanic glass (SiO2 - Silicon Dioxide)
Hardness: 5-6 (Mohs); Color: Jet black; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: Conchoidal fracture; Specific gravity: 2.4
- Hardness
- 5-6 (Mohs)
- Color
- Jet black
- Luster
- Vitreous/Glassy
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Physical properties
Hardness: 5-6 (Mohs); Color: Jet black; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: Conchoidal fracture; Specific gravity: 2.4
Formation & geological history
Formed from the rapid cooling of silica-rich (felsic) lava, which prevents crystal growth. Most specimens found today are less than a few million years old, as the glass eventually devitrifies.
Uses & applications
Used in jewelry, surgical blades (scalpels), stone tools (arrowheads), and high-end decorative ornaments.
Geological facts
Ancient cultures used obsidian to craft the sharpest blades known to man; it can produce a cutting edge many times thinner and sharper than a steel scalpel. Varieties like 'Snowflake' or 'Mahogany' obsidian contain inclusions.
Field identification & locations
Identify in the field by its distinct conchoidal (shell-like) fracture patterns and sharp edges. Commonly found in volcanic regions such as Oregon (USA), Mexico, and Iceland. Collectors look for translucency at the edges.
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