Obsidian

Physical Properties

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

Formation & Geological History

Formed from extrusive igneous activity when silica-rich felsic lava cools so rapidly that crystals do not have time to grow. Usually associated with rhyolitic volcanic eruptions from the Cenozoic era.

Uses & Applications

Used for surgical scalpels, jewelry (cabochons and beads), gemstones, and ornamental carvings. Historically used for arrowheads and cutting tools.

Geological Facts

Obsidian is not a true mineral because it lacks a crystal structure. It is often referred to as 'nature's glass'. Famous varieties include Snowflake Obsidian and Rainbow Obsidian.

Field Identification & Locations

Identify by its extremely smooth, curved (conchoidal) fracture surfaces and glassy luster. Found in areas of recent volcanic activity such as the Cascades, Iceland, Mexico, and Japan.

Identified on: 5/2/2026

Mode: Standard