Cryolite (Na3AlF6- Sodium Aluminum Fluoride) is a local mineral, only found in large quantities on the west coast of Greenland.
It’s clear or white to yellowish, but can also be black or purple. The name comes from the Greek κρύος, frost, and λίθος, stone, meaning “ice-stone” in allusion to its appearance.
It was commercially mined in large quantities in Greenland since the mid-1800’s, and used as flux for the production of aluminum. It’s a curiosity mineral.
Colourless samples seemingly ‘disappear’ in water due to very low refractive index (index of refraction is 1.338 which is close to the index of refraction of water).
Photo: Didier Descouens (Wikimedia Commons).