Rock Identifier
Obsidian (Black Obsidian) (Amorphous volcanic glass (primarily SiO2)) — igneous
igneous

Obsidian (Black Obsidian)

Amorphous volcanic glass (primarily SiO2)

Hardness: 5.0–6.0; Color: Jet black (sometimes grey or green); Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); SG: 2.3–2.6

Hardness
5
Color
Jet black (sometimes grey or green)
Luster
Vitreous/Glassy
Identified More igneous →

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Physical properties

Hardness: 5.0–6.0; Color: Jet black (sometimes grey or green); Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); SG: 2.3–2.6

Formation & geological history

Formed as an extrusive igneous rock when felsic lava extruded from a volcano cools rapidly with minimal crystal growth. Found in geologically recent volcanic areas (Cenozoic era).

Uses & applications

Historically used for making extremely sharp blades/arrowheads; currently used in jewelry (cabochons), surgical scalpel blades, and ornamental stone carvings.

Geological facts

Because it is a glass and lacks crystal structure, it breaks with conchoidal fractures, resulting in edges that are molecularly thin and sharper than high-quality steel scalpels.

Field identification & locations

Identify by its glass-like appearance, smooth curved (conchoidal) fractures, and jet-black color. Found in volcanic regions like the American West (Oregon, California), Iceland, and Mexico.

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