Obsidian

Obsidian sits in a genuinely unusual category: it is a naturally occurring glass, which means it possesses no crystalline structure whatsoever. Where every mineral is defined by a repeating three-dimensional arrangement of atoms, obsidian’s silica content cooled too quickly for that ordering to occur — the atoms are frozen in a disordered, amorphous arrangement more similar to a very stiff liquid than a true solid. This is not a technicality to brush past. It means obsidian has no cleavage planes (because cleavage requires a crystal structure to cleave along), no true melting point (it softens gradually across a temperature range, as glass does), and it fractures in the distinctive conchoidal pattern — smooth, curved, shell-like — that made it among the sharpest cutting materials available to humans before the development of metal tools. Obsidian blades have been found with edges measured at the level of a single molecule in thickness, sharper than surgical steel.

The inclusions and variants of obsidian reward close inspection. Rainbow obsidian owes its iridescent bands to thin layers of magnetite nanoparticles or gas bubbles aligned during the flow of cooling lava; the interference of light across those layers produces colours that shift dramatically as you tilt the stone. Snowflake obsidian is particularly interesting to me from a mineralogical standpoint: the white or grey star-shaped patches are clusters of cristobalite, a high-temperature polymorph of silica that has begun to crystallise out of the glassy matrix over time. This devitrification — the slow, partial conversion of glass into crystalline mineral — is what obsidian is, very gradually, always trying to do. Given enough time (geologically speaking), all obsidian would eventually become something else. There is something almost restless in that, and something honest.