Obsidian

Hardness: 5-5.5 Mohs; Color: Jet black; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: Conchoidal fracture; Specific gravity: 2.3-2.6

Formed by the extremely rapid cooling of silica-rich (felsic) lava, which prevents crystal growth. Most specimens found in the field are from the Quaternary period (less than 2.6 million years old) because glass is metastable and eventually devitrifies.

Used historically for sharp tools like arrowheads and scalpels (due to its ability to produce an edge thinner than steel); currently used in jewelry, ornamental carvings, and collectibles.

Because it lacks a crystal structure, obsidian is not technically a mineral but a mineraloid. It was a major trade commodity in ancient Mesoamerican and Mediterranean civilizations.

Identify by its characteristic conchoidal (shell-like) fracture, sharp edges, and deep black glassy appearance. Commonly found in volcanic regions such as the American West (Oregon, California), Iceland, and Mexico.