Cryolite made an excellent flux to make the process less expensive. Now it is too rare to be used for this purpose and sodium aluminum fluoride is produced artificially to fill the void. A curious note about cryolite is the fact that it has a low index of refraction close to that of water. This means that if emersed in water, a perfectly clear colorless crystal of cryolite or powdered cryolite will essentially disappear. Even a specimen of cloudy cryolite will become more transparent and its edges will be less distinct, an effect similar to ice in water except that the ice floats.
Color is clear or white to yellowish, but can also be black or purple.
Luster is vitreous. Transparency crystals are transparent to translucent. Cleavage is absent, but three parting directions produce what looks like a psuedo-cubic cleavage. Fracture is uneven. Streak is white.