Calcite occurs in a vareity of forms as seen in these samples of calcite.
Calcite can be found in a wide range of color, the most common being white and yellow, but also occurring in shades of green, blue, orange, gray, purple, red, and brown. One way to test for calcite is by using acid, such as very dilute hydrochloric acid; when dropped onto a sample containing calcite it will fizz vigorously. Some have also reported using vinegar, although it produces a weaker fizzing action. [Always use caution with acids regardless of how ‘weak’ or dilute they are.]
Some of the more unusual colors of calcite include blue, green, and peach/orange.
A neat property of calcite is its ability to produce a double refraction. This is best seen in calcite that is in a clear rhombohedral form (at one time specimens like these were called Iceland spar). Calcite is also the main constituent in shells for marine organisms, and found in many fossils. Calcite is said to have been valued by ancient peoples for building material once they realized it could be crushed and mixed with water to form cement and plaster. Calcite is sometimes used in decoration or jewelry, but must be handled with care (if not enhanced) as it is more fragile.
Calcite as it appears in its perfect rhombohedral shape (left) and the very clear optical calcite showing double refraction (right).
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Chemical composition: CaCO3
Crystal System: Trigonal
Color: Commonly white & yellow, also peach/orange, gray, blue, purple, green, red, and brown.
Habit: Crystalline, prismatic, tabular, scalenohedral, rhomobohedral, stalactitic, massive, concretions
Luster: Vitreous to pearly
Specific Gravity: ~2.7
Occurrence: Worldwide, though the UK is known for very excellent crystals, and optical calcite is collected in Iceland.
A Guide to Rocks and Fossils by B. Busbey III, R. R. Coenraads, P. Willis, and D. Roots. Published 2002 by Fog City Press. ISBN: 1877019518
Rocks, Minerals, & Fossils of the World by C. Pellant and R. Phillips. Published 1990 by Little, Brown and Co. ISBN: 0316697966
Wikipedia – Calcite. http://en.wikipedia.org/wiki/Calcite
Wikipedia – Aragonite. http://en.wikipedia.org/wiki/Aragonite