Calcite

An interesting mineral. There is a museum in Franklin, New Jersey which showcases different types of fluorescent calcite crystals which glow different colors under ultraviolet light because they contain manganese. Colorless calcite sometimes shows a cat's eye effect when cut in cabochon form; these are seen as gemstones, but generally it's considered too soft for jewelry.
A certain type of squarish calcite crystal that is transparent will sometimes make you see double when you look through it because it refracts the light passing through them twice (birefringence).

Calcite is perhaps one of the most common minerals in the world. It comprises about 4% by weight of the Earth's crust and can be found all over the world. Calcite, when found in crystals and not en masse, can be in more than 300 different forms. Most often, calcite is found in rhombohedral, prismatic or scalenohedral forms. However, over 1,100 shapes of calcite have been seen.

Calcite, which gets its name from the Greek word for lime, chalix, is commonly found in white, yellow, pink, or clear crystals. The pink color sometimes found is from concentrations of manganese in the stone. Clear calcite which is highly doubly refractive is called Iceland Spar, as it was first found in Iceland. Because calcite is so common, it has several different names and types, such as bruyerite, hematoconite, manganoan calcite, onyx-marble, pelagosite, prasochrome, and satin spar. Calcite will often be flourescent when placed underneath a black light or other source of ultra-violet light.

Calcite is formed inside hydrothermal vents, as well as in sedimentary limestones and speleothemes in caves. Rarely it will be found in syenitic or pegmatitic igneous rocks. Calcite is also a large constituent of marble, and can be found in several other rocks and minerals. When in other rocks and minerals, calcite is often white and opaque. Geographically, calcite is found all over the world, Brazil, Germany, Iceland, Cornwall, Bombay, the Harz mountains, Missouri, Oklahoma, Kansas and Wisconsin, to name a few locations.

Sources:
Precious Stones, by Dr. Max Bauer. Charles E. Tuttle Company: Rutland Vermont and Tokyo, Japan, 1969
Gemstones of the World, by Walter Schumann. Sterling Publishing Co., New York, 1979
Simon and Schuster's guide to Rocks and Minerals, Simon and Schuster Inc. New York, 1978
http://mineral.galleries.com/minerals/carbonat/calcite/calcite.htm
http://www.mindat.org/min-859.html

Cal"cite (?), n. [L. calx, calcis, lime.] Min.

Calcium carbonate, or carbonate of lime. It is rhombohedral in its crystallization, and thus distinguished from aragonite. It includes common limestone, chalk, and marble. Called also calc-spar and calcareous spar.

⇒ Argentine is a pearly lamellar variety; aphrite is foliated or chalklike; dogtooth spar, a form in acute rhombohedral or scalenohedral crystals; calc-sinter and calc-tufa are lose or porous varieties formed in caverns or wet grounds from calcareous deposits; agaric mineral is a soft, white friable variety of similar origin; stalaclite and stalagmite are varieties formed from the drillings in caverns. Iceland spar is a transparent variety, exhibiting the strong double refraction of the species, and hence is called doubly refracting spar.

© Webster 1913.