igneous
Obsidian
Obsidian (Amorphous volcanic glass)
Hardness: 5–6 Mohs; Color: Black, sometimes dark brown or grey; Luster: Vitreous (glassy); Structure: Amorphous (non-crystalline); Fracture: Conchoidal; Specific Gravity: 2.3–2.6.
- Hardness
- 5–6 Mohs
- Color
- Black, sometimes dark brown or grey
- Luster
- Vitreous (glassy)
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Physical properties
Hardness: 5–6 Mohs; Color: Black, sometimes dark brown or grey; Luster: Vitreous (glassy); Structure: Amorphous (non-crystalline); Fracture: Conchoidal; Specific Gravity: 2.3–2.6.
Formation & geological history
Formed through the rapid cooling of felsic lava, which prevents the growth of crystals. Typically found in volcanic regions of various ages, from the Holocene to the Tertiary period.
Uses & applications
Historically used for weapons (arrowheads, blades). Currently used for surgical scalpels, decorative jewelry, gemstones, and as an ornamental stone in construction.
Geological facts
Obsidian is technically a supercooled liquid rather than a true mineral because it lacks a crystalline structure. It was highly traded in ancient civilizations as a precious commodity.
Field identification & locations
Identify in the field by its distinctive glassy appearance and sharp, curved (conchoidal) fracture edges. Commonly found in areas with recent rhyolitic volcanic activity like Iceland, Mexico, and the Western USA.
