pyrophane

from The Century Dictionary.

• noun 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.

from the GNU version of the Collaborative International Dictionary of English.

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

from Wiktionary, Creative Commons Attribution/Share-Alike License.

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