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Encyclopedic overview
A sand grain of volcanic glass under the petrographic microscope. Its amorphous nature makes it disappear in cross-polarized light (bottom frame). The scale box is in millimeters.
Volcanic glass is the amorphous (non-crystalline) product of rapidly cooling magma. Like all types of glass, it is a state of matter intermediate between the closely packed, highly ordered array of a crystal and the highly disordered array of liquid. Volcanic glass may refer to the interstitial material, or matrix, in an aphanitic (fine-grained) volcanic rock, or to any of several types of vitreous igneous rocks.
Excerpted from Wikipedia’s “volcanic glass” article, available under the CC BY-SA 4.0 licence.