pyrophane
Mineralogy (now rare )
noun
A mineral said to become translucent or to show colour when warmed, especially a form of opal with this property, made by impregnating hydrophane with wax; (also, partly by confusion) any of several other varieties of opal.
Origin
Late 18th century; earliest use found in Richard Kirwan (1733–1812), chemist and mineralogist. From pyro- + -phane.