Optical Calcite, China

Calcite is one of the most common minerals which grows in a wide variety of different shapes, colors and translucencies. The mineral itself actually belongs to a calcite group of "related carbonates" that are "isomorphous" with each other. All members of the group crystalize in triangular formations, have perfect rhombohedron cleavages, and in transparent rhombohedrons, display strong double refractions. Aragonite and Calcite are similar in chemical composition (polymorphs) but are structured differently, with Calcite forming triangular crystallization and Aragonite forming orthorhombic crystals (three axes planes, at unequal lengths, and all three axes 90 degrees to one another. Unique Calcite formations are created inside the caves where calcium rich water penetrates the cavern's limestone enclosure and slowly drips and flows on top of itself, allowing for the formation of unique stalactite and other globular formations to be created. In this slow process, other minerals can intermix with each other, adding to the interest and forming unusual specimens, such as the one displayed.

Optical Calcite is a birefringent crystal, which means that it refracts light into two separate rays, and moving at two different speeds. If you hold Optical Calcite over an object and look through the crystal, you will see the object doubled.