igneous
Obsidian
Obsidian (Volcanic Glass, SiO2 + impurities)
Hardness: 5-5.5 on Mohs scale; Color: typically jet black but can be dark green or brown; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: None (Conchoidal fracture); Specific Gravity: 2.4
- Hardness
- 5-5
- Color
- typically jet black but can be dark green or brown
- Luster
- Vitreous/Glassy
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Physical properties
Hardness: 5-5.5 on Mohs scale; Color: typically jet black but can be dark green or brown; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: None (Conchoidal fracture); Specific Gravity: 2.4
Formation & geological history
Formed through the rapid cooling of felsic (silicic) lava, which prevents the growth of mineral crystals. It is found in volcanic regions where cooling was extremely fast, such as at the edges of basaltic lava flows or when lava enters water.
Uses & applications
Used historically for cutting tools, mirrors, and arrowheads due to its sharp edges. Modern uses include surgical scalps (for precision), jewelry, lapidary arts, and ornamental decoration.
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' rather than a mineral because it lacks a crystalline structure.
Field identification & locations
Identify by its glass-like appearance, smooth texture, and curved, shell-like (conchoidal) fractures. Found in volcanic areas throughout the Americas, Iceland, and Italy. Collectors look for unique varieties like Snowflake or Mahogany obsidian.
