Black Obsidian

Physical Properties

Hardness: 5-5.5 (Mohs scale); Color: Black, sometimes dark brown; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: Conchoidal fracture; Specific gravity: 2.3-2.6

Formation & Geological History

Formed through the rapid cooling of silica-rich (felsic) lava, which prevents crystal growth. It is relatively young in geological terms because it eventually breaks down or devitrifies into crystals.

Uses & Applications

Used for making surgical scalpels (sharper than steel), omamental jewelry, gemstone carvings, and historical tools like arrowheads and knives.

Geological Facts

Obsidian can be fractured to create an edge thinner and sharper than a surgeon's steel scalpel. The black color usually comes from microscopic inclusions of magnetite or hematite.

Field Identification & Locations

Identify by its characteristic conchoidal (shell-like) fracture and sharp edges. It is common in volcanic regions like the American West (Iceland, Mexico, Japan). Collectors should look for smooth, glassy surfaces.

Identified on: 4/13/2026

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