Obsidian looks like it should be classified alongside quartz points and calcite crystals, dark, glassy, occasionally faceted-looking where it’s broken. It isn’t a mineral at all, at least not by the strict definition. It’s volcanic glass, and the reason comes down to how fast it cooled rather than what it’s made of.
Too fast to crystallize
Minerals form when atoms have time to arrange themselves into an orderly, repeating lattice as a melt cools. Obsidian forms from silica-rich lava that cools so quickly, sometimes in a matter of seconds to minutes at a lava flow’s outer edge or where it meets water, that the atoms never get the chance to organize into any crystal structure at all. They freeze in place in a disordered, glass-like arrangement instead, which is why obsidian has no cleavage planes, no crystal faces, and no consistent internal symmetry to speak of, the opposite condition from the seven crystal systems that organize true minerals.
What breaking it reveals
Without a crystal lattice to guide a break along predictable planes, obsidian fractures the same way ordinary glass does: conchoidal fracture, smooth, curved surfaces radiating outward from the point of impact, sharp enough at the edge that obsidian was a preferred toolmaking material across multiple ancient cultures long before metal tools existed. Our piece on conchoidal fracture goes further into why that particular break pattern shows up in both glass and certain true crystalline minerals like quartz.
Chemically, obsidian and granite can start from nearly the same silica-rich magma. The only real difference between a mountain of crystalline granite and a chunk of obsidian glass is how much time the melt had to cool. Given enough time, it’s rock. Given almost none, it’s glass.
