When the Vikings first visited the west coast of Greenland they found that the Eskimos who lived there used heavy stones to anchor their boats.
The Vikings were amazed to see that when these stones were dropped in the water, they became nearly invisible. Today this mineral is called Cryolite. Greenland is the only place it is found. When pieces of this mineral are seen out of water, they look like ordinary white stone.
What is remarkable about cryolite is its translucence, light passes through it easily. Thus, when a piece of this stone is placed in water, it becomes almost invisible.
When you look in the water, you see the water, but you don’t see the stone!
Cryolite was used to extract aluminum from the ore bauxite. It is now too rare to be used for this purpose.