perlitic

The non-spherulitic glassy portion is sometimes perlitic, and these rocks are always brittle.

Perlitic structure, also common in glasses, consists of the presence of concentric rounded cracks owing to contraction on cooling.

Some of these rocks show perlitic or spherulitic structure, and such rocks were probably originally glassy (obsidians or pitchstones), but by lapse of time and processes of alteration have slowly passed into very finely crystal-line state.