igneous
Obsidian
Obsidian (volcanic glass), primarily SiO2 (silicon dioxide)
Hardness: 5-5.5 (Mohs scale). Color: Typically jet black, but can be dark green, brown, or gray. Luster: Vitreous/glassy. Crystal Structure: Amorphous (non-crystalline). Cleavage: None (conchoidal fracture). Specific Gravity: 2.3-2.6.
- Hardness
- 5-5
- Color
- Typically jet black, but can be dark green, brown, or gray
- Luster
- Vitreous/glassy
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Physical properties
Hardness: 5-5.5 (Mohs scale). Color: Typically jet black, but can be dark green, brown, or gray. Luster: Vitreous/glassy. Crystal Structure: Amorphous (non-crystalline). Cleavage: None (conchoidal fracture). Specific Gravity: 2.3-2.6.
Formation & geological history
Formed through the rapid cooling of silica-rich (felsic) lava, which prevents crystal growth. It is found in geologically young volcanic areas, mostly from the Pliocene to the Holocene era, as it devitrifies over millions of years.
Uses & applications
Historically used for arrowheads and knives. Today, it is used for surgical scalpel blades, decorative jewelry, cabochons, and as a semi-precious gemstone for collectors.
Geological facts
Obsidian breaks with a conchoidal fracture, creating edges that are thinner and sharper than high-quality steel scalpels. It is technically a mineraloid because it lacks a crystalline structure.
Field identification & locations
Identify in the field by its smooth, glassy surface and razor-sharp edges if broken. Commonly found in volcanic regions like the American West (Iceland, Mexico, Japan). Collectors look for 'Snowflake' or 'Rainbow' varieties.