Cryolite

Cryolite is a rare mineral that consists of sodium fluoride and aluminium fluoride bonded. It is used in the production of aluminium. It is first melted and then aluminium oxide is dissolved in it. Then the solution is electrolyzed. If it is immersed in water, it becomes invisible although it does not dissolve. It used to be found in large quantities in Greenland, but it has been all used up. Now it is made from fluorite artificially.