any of a type of microscopic body occurring in such glassy igneous rocks as obsidian and pitchstone. Crystallites are regarded as incipient or embryonic crystals, though they often have no recognizable crystallographic form and are too small to polarize light. They occur when magma (molten rock material) congeals so rapidly that crystallization remains incomplete. Crystallites are distinguished from microlites, which are slightly larger forms recognizable as mineral species.

There are several varieties of crystallites, and names have been assigned to indicate their particular shapes. Globulites, for example, are oval or spherical; scopulites may be feathery or flowerlike. The faster-growing faces of a crystallite become smaller, so that the slower-growing faces are the longer ones. Rodlike crystallites composed of a number of smaller elongate forms are called bacillites (bacillite). Belonites are elongated with pointed or rounded ends; they include the forms called longulites (elongated), spiculites (tapered toward both ends), and clavalites (dumbbell-shaped).