pyrophane

Century Dictionary and Cyclopedia

1. n. A kind of opal (hydrophane) which by the absorption of melted wax is rendered translucent so long as it is hot, but becomes opaque again on cooling.

Wiktionary

1. n. mineralogy A mineral which is opaque in its natural state, but is said to change its color and become transparent by heat.

GNU Webster's 1913

1. n. (Min.) A mineral which is opaque in its natural state, but is said to change its color and become transparent by heat.