igneous
Obsidian
Volcanic Glass (predominantly SiO2, approximately 70% or more)
Hardness: 5-5.5 on Mohs scale; Color: typically jet black, can be dark green or brown; Luster: Vitreous (glassy); Crystal Structure: Amorphous (non-crystalline); Cleavage: None (exhibits prominent conchoidal fracture); Specific Gravity: 2.35-2.6
- Hardness
- 5-5
- Color
- typically jet black, 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, can be dark green or brown; Luster: Vitreous (glassy); Crystal Structure: Amorphous (non-crystalline); Cleavage: None (exhibits prominent conchoidal fracture); Specific Gravity: 2.35-2.6
Formation & geological history
Formed through the rapid cooling of felsic lava, which prevents the growth of crystals. It is geologically young (usually Cenozoic) as it eventually hydrates and devitrifies into other materials. Found in areas with recent volcanic activity.
Uses & applications
Used in the manufacture of surgical scalpels for ultra-fine cutting, experimental high-end cutlery, jewelry (cabochons and beads), and historically for arrowheads, blades, and ornamental mirrors.
Geological facts
Obsidian is technically a 'mineraloid' rather than a true mineral because it lacks a crystalline structure. It was highly prized in the Stone Age for its ability to produce edges thinner and sharper than the best steel.
Field identification & locations
Identify in the field by its distinct glassy appearance and shell-like curved (conchoidal) fracture lines. Common locations include Oregon, California, Iceland, Mexico, and Japan. Collectors should look for sharp edges and transparency on thin margins.
