The Collaborative International Dictionary
Pyrophane
Pyrophane \Pyr"o*phane\, n. [See Pyrophanous.] (Min.) A mineral which is opaque in its natural state, but is said to change its color and become transparent by heat.
Wiktionary
pyrophane
n. (context mineralogy English) A mineral which is opaque in its natural state, but is said to change its color and become transparent by heat.