Obsidian (likely black volcanic glass)

Black, opaque, vitreous luster with sharp conchoidal-fracture edges; hardness about 5–5.5 Mohs, no cleavage, noncrystalline glassy structure, and specific gravity roughly 2.3–2.6.

Forms when silica-rich lava cools so rapidly that crystals cannot grow, typically during Quaternary volcanic eruptions. Common sources include volcanic regions of the western United States, Mexico, Iceland, Armenia, and Italy.

Used historically for blades and arrowheads, and today for ornaments, cabochons, sculptures, and collector specimens; it has limited construction use.

Its edges can be sharper than surgical steel, though they are brittle and easily chipped. A slight translucence at thin edges and conchoidal fracture help distinguish it from black slag or glass.

Identify by its natural irregular form, glassy luster, curved fracture surfaces, and absence of bubbles or flow textures typical of manufactured glass; exact identification may require testing. Small common fragments are worth about $1–$10; attractive snowflake, rainbow, or large polished specimens may sell for $20–$200+.