igneous

Obsidian (Black Obsidian)

Volcanic Glass (Primary composition: SiO2 - Silicon Dioxide)

Hardness: 5–5.5 Mohs; Color: Jet black, opaque to translucent at edges; Luster: Vitreous (glassy); Crystal Structure: Amorphous (none); Cleavage: None (conchoidal fracture); Specific Gravity: 2.4

Hardness: 5–5
Color: Jet black, opaque to translucent at edges
Luster: Vitreous (glassy)

Identified March 25, 2026More igneous →

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Physical properties

Hardness: 5–5.5 Mohs; Color: Jet black, opaque to translucent at edges; Luster: Vitreous (glassy); Crystal Structure: Amorphous (none); Cleavage: None (conchoidal fracture); Specific Gravity: 2.4

Formation & geological history

Formed from the rapid cooling of felsic lava, preventing crystal growth. Usually associated with geologically young volcanic activity (Cenozoic era) due to its tendency to devitrify over time.

Uses & applications

Used for making scalpel blades (sharper than steel), jewelry/cabochons, ornamental carvings, and historically for arrowheads and scrapers.

Geological facts

Because it has no crystal structure, its edges can reach nearly molecular thinness, making it one of the sharpest objects known. It was highly prized in pre-Columbian Mesoamerica for both utility and ritual.

Field identification & locations

Field identification: Look for extreme glassiness and distinctive shell-like (conchoidal) curved fractures. Found globally in volcanic regions like the American West, Iceland, Mexico, and Italy.

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