Aphanite

Aphanite, or aphanitic as an adjective, (from the Greek αφανης, invisible) is a name given to certain igneous rocks which are so fine-grained that their component mineral crystals are not detectable by the unaided eye (as opposed to phaneritic igneous rocks, where the minerals are visible to the unaided eye). This texture results from rapid cooling in volcanic or hypabyssal (shallow subsurface) environments. As a rule, the texture of these rocks is not quite the same as that of volcanic glass (e.g. obsidian), with volcanic glass being even finer grained (or more accurately, non-crystalline) than aphanitic rocks, and having a glass-like appearance.