Obsidian

Hardness: 5-6 (Mohs scale), Color: Jet black to dark gray, Luster: Vitreous (glassy), Structure: Amorphous (non-crystalline), Cleavage: None (conchoidal fracture), Specific Gravity: 2.35-2.6

Formed from rapid cooling of silica-rich (felsic) lava, which prevents crystal growth. Usually found in volcanic regions from the Quaternary period to relatively recent eruptions because glass eventually devitrifies.

Used historically for sharp edges, tools, and arrowheads. Modern uses include surgical scalpels, decorative jewelry, and collectible specimens.

Because it has no crystal structure, obsidian can be fractured to a thickness of only a few molecules, making it sharper than even high-quality steel scalpels. Ancient Aztecs used it for sacrificial knives and mirrors.

Identify by its glass-like texture and characteristic conchoidal (shell-like) fracture patterns. Commonly found in places like Oregon, Iceland, and Mexico. Collectors look for unique varieties like Snowflake or Mahogany obsidian.