Obsidian

Physical Properties

Hardness: 5-5.5 (Mohs scale); Color: Black, dark gray, or dark brown; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.4

Formation & Geological History

Formed through the rapid cooling of silica-rich (felsic) lava, which prevents the growth of mineral crystals. It is typically found in volcanic areas of recent geological age (Tertiary and Quaternary).

Uses & Applications

Historically used for edged tools like arrowheads and scalpels. Currently used in jewelry (beads/cabochons), ornamental sculpture, and high-quality surgical blades.

Geological Facts

Because it is a glass and lacks a crystal structure, obsidian is chemically unstable over millions of years and eventually devitrifies into fine-grained quartz and feldspar crystals.

Field Identification & Locations

Identify by its glass-like texture and sharp, curved (conchoidal) fractures. Commonly found in volcanic regions like the American West, Mexico, Iceland, and Italy. Collectors look for unique variations like 'Snowflake' or 'Mahogany' obsidian.

Identified on: 4/17/2026

Mode: Standard