Rock Identifier
Black Obsidian (Volcanic Glass (Amorphous Silicon Dioxide SiO2)) — Igneous
Igneous

Black Obsidian

Volcanic Glass (Amorphous Silicon Dioxide SiO2)

Hardness: 5-5.5 on Mohs scale; Color: Jet black to dark gray; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.35-2.6

Hardness
5-5
Color
Jet black to dark gray
Luster
Vitreous/Glassy
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Physical properties

Hardness: 5-5.5 on Mohs scale; Color: Jet black to dark gray; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.35-2.6

Formation & geological history

Formed from rapid cooling of felsic (silicic) lava, preventing crystal growth. Usually found in margin areas of rhyolitic lava flows. Geological age varies by location, typically younger than a few million years as it eventually devitrifies.

Uses & applications

Used for making surgical scalpel blades (sharper than steel), ornamental jewelry, decorative carvings, and meditation stones. Historically used for arrowheads and scrapers.

Geological facts

Obsidian fractures with extremely sharp edges, even down to molecular thickness. It was highly prized by the Aztecs for making 'macuahuitl' (club-swords) and mirrors.

Field identification & locations

Identify by its glass-like appearance and characteristic conchoidal (shell-like) fracture patterns. Commonly found in volcanic regions like the American West (Oregon, California), Mexico, Iceland, and Italy. Often found as rounded 'Apache Tears'.

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