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Iceland Spar

Calcite - Double refraction

The clear form of Calcite is called Iceland Spar. This piece is not a natural crystal. It has been split into this shape. Many minerals split more easily along in some ways than others. These are called 'cleavage planes'. Calcite cleaves to form this shape, called a rhombohedron.
Transparent calcite has an odd property called 'double refraction'. You can see through it, but you see two images rather than just one. Here, we are looking at the word "calcite" through it. You can see both images, and a rainbow. If you turn the calcite round, then one image stays the same, while the other moves round it. (You can't show this in a photograph.) Double refraction
Larger picture of double refraction: