Obsidian

Hardness (Mohs scale): 5.0–6.0, Color: Black/dark brown, Luster: Vitreous/glassy, Crystal structure: Amorphous (non-crystalline), Cleavage: None (conchoidal fracture), Specific gravity: 2.35–2.6

Formed through the rapid cooling of silica-rich (felsic) lava, preventing crystal growth. Usually found in margin of lava flows or where lava contacts water. Most specimens are younger than Cretaceous age as glass eventually devitrifies.

Used historically for sharp tools and weapons (arrowheads/knives); currently used for jewelry, surgical scalpel blades, and ornamental purposes.

Unlike almost all other rocks, obsidian is not made of minerals; it is a natural glass. It is often sharper than high-quality steel surgical scalpels because its edge can reach molecular thinness.

Identify by its characteristic conchoidal fracture (curved, shell-like ripples) and glassy luster. Commonly found in volcanic regions like the American West, Iceland, Italy, and Mexico.