Igneous

Obsidian

Volcanic Glass (Primary composition: SiO2 - Silicon Dioxide)

Hardness: 5.0–6.0; Color: Jet black (often translucent on thin edges); Luster: Vitreous (glassy); Crystal Structure: Amorphous (none); Cleavage: None/Conchoidal fracture; Specific Gravity: 2.35–2.6.

Hardness: 5
Color: Jet black (often translucent on thin edges)
Luster: Vitreous (glassy)

Identified March 20, 2026More igneous →

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Physical properties

Formation & geological history

Obsidian is produced when felsic lava extruded from a volcano cools rapidly with minimal crystal growth. While it occurs throughout geologic time, most found today is relatively young (less than few million years) because it eventually devitrifies into crystals.

Uses & applications

Used in ornamental jewelry, modern surgical scalpel blades (due to its ability to be fractured to a molecularly thin edge), and historical tool-making (arrowheads and scrapers).

Geological facts

Because it has no crystal structure, obsidian is not technically a mineral but a 'mineraloid.' It was one of the most important commodities in the prehistoric world for manufacturing sharp cutting tools.

Field identification & locations

Identify by its glass-like appearance, very smooth surface when polished, and characteristic curved (conchoidal) fractures. Found in volcanic regions like the American West, Iceland, and Mexico.

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