Obsidian

Hardness: 5-6 (Mohs); Color: Black; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Fracture: Conchoidal; Specific Gravity: 2.3-2.6

Forms when felsic lava extruded from a volcano cools very rapidly with minimal crystal growth. It is found in volcanic regions of any geological age, though most available specimens are relatively young (Cenozoic) because the glass eventually devitrifies.

Historically used for stone tools and weapons (arrowheads/knives). Modern uses include ornamental jewelry, scalpel blades for surgery, and collector specimens.

Obsidian fractures with extremely sharp edges, much sharper than high-quality steel scalpels. Unlike most rocks, it has no crystal structure and is technically a mineraloid.

Identified by its jet-black color and characteristic conchoidal (shell-like) fracture patterns. Frequently found in areas with recent volcanic activity like Oregon, California, Iceland, and Mexico.