Cryolite is a rare sodium aluminofluoride (Na3AlF6). Its name ("ice-rock") refers to its ~resemblance to ice in terms of luster and low index of refraction (= a measure of how light passes through a material, in terms of how much the light gets bent and the light’s change of speed). Cryolite has a nonmetallic luster, a clear to white color, cubic parting (not cubic cleavage), and a dull glassy luster.
Cryolite used to have considerable economic significance, as it was used as a flux in making aluminum metal. The only exploitable cryolite occurrence on Earth is (was) in southern Greenland's Gardar Igneous Province at Ivigtut (aka Ivittuut). Ivigtut cryolite was first mined in the 1850s, and that deposit is now ~exhausted. Aluminum metal production nowadays uses artificially-made sodium aluminofluoride flux.
Cryolite (7.1 cm across) from Ivigtut, southwestern Greenland, the only significant cryolite occurrence on Earth. This deposit is a cryolite body of igneous origin within the roof zone of the Ivigtut Granite (mid-Mesoproterozoic, 1.171 billion years), a pipe-shaped intrusion penetrating an intrusion breccia unit. In addition to cryolite, ten other very rare aluminofluoride minerals have been reported from the Ivigtut Deposit.
Some info. from:
Pauly & Bailey (1999) - Genesis and evolution of the Ivigtut cryolite deposit, SW Greenland. Meddelelser om Grønland, Geoscience 37. 60 pp.