Obsidian

Hardness: 5-5.5 (Mohs); Color: Jet black to dark grey (can be green, brown, or red); Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.35-2.6.

Formed by the rapid cooling of felsic (silica-rich) lava, which prevents crystals from growing. It is typically found in geologically young volcanic regions (Quaternary-Neogene) as it eventually hydrates and devitrifies into other minerals.

Historically used for making extremely sharp blades, scrapers, and arrowheads. Modern usage includes decorative jewelry (cabochons), precision surgical scalpels, and home decor items.

Obsidian is technically a 'mineraloid' rather than a true mineral because it lacks a crystalline structure. Ancient civilizations like the Aztecs and Maya valued it as a status symbol and used it for ritual mirrors and weapons.

Identify in the field by its characteristic smooth, curved (conchoidal) fracture surfaces that resemble broken glass and its sharp edges. It is commonly found in volcanic areas such as the western United States (Oregon, California, Iceland, and Mexico). Collectors look for a smooth, matte exterior often weathered into a 'desert varnish' or river-worn cobble shape.