igneous
Obsidian
Volcanic Glass (SiO2)
Hardness: 5–5.5 (Mohs), Color: Typically black but can be green or brown, Luster: Vitreous/Glassy, Crystal Structure: Amorphous (non-crystalline), Cleavage: None (conchoidal fracture), Specific Gravity: 2.35–2.6
- Hardness
- 5–5
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Physical properties
Hardness: 5–5.5 (Mohs), Color: Typically black but can be green or brown, 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. Predominantly found in volcanic areas from various geological ages, primarily the Cenozoic era.
Uses & applications
Used historically for sharp tools/weapons (scalpels, arrowheads), in jewelry (beads, cabochons), and as an ornamental stone in modern decor.
Geological facts
Obsidian breaks with a conchoidal fracture, creating edges thinner than even the best steel scalpels. It is sometimes used in specialized eye surgeries today because it causes less tissue damage than metal blades.
Field identification & locations
Identify by its intense glassy sheen and characteristic conchoidal (curved, shell-like) fractures. Common in volcanic regions like the American West (Glass Buttes), Iceland, and Italy. Collectors look for unique varieties like Snowflake or Mahogany obsidian.
