This specimen exhibits double refraction and the characteristic shape of nearly perfect calcite crystals, a cube that looks as if it's been skewed to one side. The internal arrangement of atoms that leads to this external shape is called rhombohedral . Beautiful as this crystal seems, calcite is not used as a gemstone because it is too soft and scratches easily. It's hardness on the Moh's scale is 3. Since natural calcite crystals most often assume many other looks, please check out the range of variation by referring to other images.
Many organisms create their shells out of the mineral calcite.
Photo kindly provided by Gemhut. All rights reserved.