Obsidian

Physical Properties

Hardness: 5-5.5 Mohs; Color: Jet black; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: Conchoidal fracture; Specific gravity: 2.4

Formation & Geological History

Formed from felsic lava that cools so rapidly that crystals do not have time to grow. It is common in areas of recent volcanic activity (Cenozoic era/Quaternary period).

Uses & Applications

Used for making scalpel blades (sharper than steel), jewelry, ornamental carvings, and historical tool-making (arrowheads/knives).

Geological Facts

Obsidian is technically a 'mineraloid' because it lacks a crystal structure. Its edges can reach a thinness of nearly one molecule, making it remarkably sharp.

Field Identification & Locations

Look for a glassy texture and distinctive shell-like (conchoidal) fractures. Found in volcanic regions like Iceland, Mexico, and the Western United States (Oregon/California).

Identified on: 4/28/2026

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