Volcanic glass is the
amorphous (uncrystallized) product of rapidly cooling
magma. Like all types of
glass, it is a state of matter intermediate between the close-packed, highly ordered array of a crystal and the highly disordered array of gas. Volcanic glass can refer to the interstitial, or
matrix material in an
aphanitic (fine grained) volcanic rock or can refer to any of several types of vitreous
igneous rocks. Most commonly, it refers to
obsidian, a
rhyolitic glass with high
silica content.