Black obsidian

Hardness about 5–5.5 Mohs; opaque black with vitreous luster and conchoidal fracture. It lacks a crystal structure and cleavage; specific gravity is roughly 2.35–2.65.

Forms when silica-rich lava cools rapidly, preventing crystal growth, typically during Quaternary volcanic eruptions. Common sources include Mexico, the western United States, Armenia, Iceland, and Japan.

Used for cabochons, beads, carvings, decorative objects, and historical tools or blades. This appears to be a polished or tumbled jewelry specimen.

Obsidian can produce exceptionally sharp edges when fractured, sharper than many steel blades at the microscopic level. Rainbow, snowflake, sheen, and mahogany varieties result from inclusions or optical effects.

Identify it by its glassy luster, smooth curved fracture surfaces, uniform dark color, and lack of visible grains; it is lighter and more glass-like than basalt. The exact identity is image-based, so black tourmaline or polished industrial glass are alternatives requiring testing.