Obsidian is amorphous volcanic glass — rapidly cooled silica-rich lava that never had time to crystallize. It has no cleavage, no crystal structure, and no mineral classification, but it belongs in every field guide because collectors encounter it constantly in volcanic regions and it behaves unlike any true mineral. The signature: conchoidal fracture producing curved, shell-like breaks with edges sharp enough to have been used as surgical blades. Black or dark brown is typical, though some specimens flash iridescent gold, silver, or rainbow sheens, and "snowflake obsidian" contains white cristobalite inclusions. Despite its dark color, obsidian's chemistry is felsic — closer to granite than basalt.
Obsidian Identification Sheet
Volcanic glass that looks like a mineral but technically isn't — and produces the sharpest natural edges known.
Streak
White
White (rarely tested — focus on fracture)
Luster
Vitreous
Vitreous (glassy)
CleavageNone — amorphous glass; conchoidal fracture produces sharp, curved shell-like surfaces
Common lookalikes
Black tourmaline (crystals, striated); dark basalt (granular, dull fracture); jet (organic, lighter). The test: hold it to light — obsidian transmits a reddish glow at thin edges; basalt does not.