igneous
Obsidian
Obsidian (Amorphous volcanic glass, SiO2 with impurities)
Hardness: 5-5.5 (Mohs scale); Color: Black, deep brown, or grey; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: None (shows conchoidal fracture); Specific Gravity: 2.3-2.6
- Hardness
- 5-5
- Color
- Black, deep brown, or grey
- Luster
- Vitreous/Glassy
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Physical properties
Hardness: 5-5.5 (Mohs scale); Color: Black, deep brown, or grey; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: None (shows 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 appears in volcanic regions from various geological eras, primarily the Cenozoic era due to its susceptibility to devitrification over time.
Uses & applications
Used historically for cutting tools, scalpels, and arrowheads. Modern uses include surgical blades (due to its molecular edge sharpness), jewelry (cabochons/beads), and ornamental carvings.
Geological facts
Because it is a glass and not a crystal, it is technically a 'mineraloid.' It is so sharp that its edge can be thinner than a high-quality steel scalpel. Native Americans traded this extensively across North American trade routes.
Field identification & locations
Identify in the field by its characteristic 'conchoidal' (shell-like) fracture patterns and extremely sharp edges. Commonly found in Oregon, California, Iceland, and Mexico near rhyolitic volcanic sites. Collectors should look for unique sheen or 'snowflakes'.
