Tachylite (also spelled tachylyte) is a
vitreous form of
basaltic volcanic glass. This "glass" is formed naturally by the rapid cooling of molten basalt. It is a basic type of
igneous rock that is decomposable by
acids and readily fusible. The color is a black or dark-brown, and it has a greasy-looking, resinous
luster. It is very brittle and occurs in
dikes, veins and
intrusive masses. The word originates from the Greek
takhus meaning swift.