... a single naturally-occuring crystal of calcite. This unusually clear and perfect crystal form of calcite is called Iceland spar. This specimen exhibits double refraction. The characteristic shape of nearly perfect calcite crystals is a box 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. See 3-D Chime model.
As beautiful as this crystal seems, calcite is not used as a gemstone because it is too soft and scratches easily. Its hardness on the Moh's scale is 3. Natural calcite crystals often look very different so it's important to look at a variety of other images to get a feel for the look of this mineral.
Many organisms create their shells out of the mineral calcite. Many other organisms produce an aragonite shell that alters to calcite over time. Thus fossils are often composed of calcite.
Photo kindly provided by Gemhut. Images from Gemhut can be viewed at Gemhut.