igneous
Obsidian
Obsidian (Volcanic Glass)
Mohs hardness: 5.0–6.0. Color: Black to dark grey. Luster: Vitreous/glassy. Crystal structure: Amorphous (non-crystalline). Cleavage: None, exhibits prominent conchoidal (shell-like) fracture. Specific gravity: 2.35–2.6.
- Hardness
- 5
- Color
- Black to dark grey
- Luster
- Vitreous/glassy
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Physical properties
Mohs hardness: 5.0–6.0. Color: Black to dark grey. Luster: Vitreous/glassy. Crystal structure: Amorphous (non-crystalline). Cleavage: None, exhibits prominent conchoidal (shell-like) fracture. Specific gravity: 2.35–2.6.
Formation & geological history
Formed by the rapid cooling of silica-rich felsic lava, which prevents crystal growth. It is typically found in geologically recent volcanic areas (Cenozoic era) because it is metastable and eventually devitrifies into fine-grained crystals.
Uses & applications
Historically used for making sharp tools like arrowheads and knives. Modern uses include surgical scalpel blades, ornamental jewelry, and gemstone carving.
Geological facts
Obsidian is technically a 'mineraloid' rather than a mineral because it lacks a crystal structure. It breaks with extremely sharp edges—thinner and sharper than a steel scalpel—which allows for cleaner incisions and faster healing in specialized surgeries.
Field identification & locations
Identify in the field by its distinct glassy appearance and characteristic conchoidal fracture patterns. Commonly found in volcanic regions like the American West, Mexico, Iceland, and Italy. Look for smooth, curved surfaces where the rock has chipped.
