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| A Gemstone That Looks Like Deconstructed Chocolate Bars. Photo credit: Rob Lavinsky/iRocks.com |
Chocolate calcite
Resembling an artful edible decoration from some top Swiss chocolate, Incredibly deep brown crystals that are thoroughly infused with microscopic hematite grains, giving them a deep ruddy red color. This is a classic style for Tsumeb. Measuring 9 x 9 x 4.3 cm, large speciments of tis nature are very rare.
Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate. The Mohs scale of mineral hardness, based on scratch hardness comparison, defines value 3 as "calcite".
Calcite is a common constituent of sedimentary rocks, limestone in particular, much of which is formed from the shells of dead marine organisms. Approximately 10% of sedimentary rock is limestone.
Other polymorphs of calcium carbonate are the minerals aragonite and vaterite. Aragonite will change to calcite over timescales of days or less at temperatures exceeding 300°C, and vaterite is even less stable.
The carbonate minerals calcite, aragonite, and dolomite have been calculated to make up approximately 15 percent of the Earth’s sediments and sedimentary rocks and about 2 percent of the terrestrial crust. A large percentage of the calcite, the most abundant of these carbonate minerals, occurs in limestones, which constitute noteworthy proportions of many sequences of marine sediments. Calcite is also the chief component of marls, travertines, calcite veins, most speleothems (cave deposits), many marbles and carbonatites, and some ore-bearing veins.
Calcite is colourless or white when pure but may be of almost any colour—reddish, pink, yellow, greenish, bluish, lavender, black, or brown—owing to the presence of diverse impurities. It may be transparent, translucent, or opaque. Its lustre ranges from vitreous to dull; many crystals, especially the colourless ones, are vitreous, whereas granular masses, especially those that are fine-grained, tend to be dull. Calcite is number 3 on the Mohs hardness scale; thus, it can be scratched readily by a knife blade or geologic pick. It has a specific gravity of 2.71. Three perfect cleavages give calcite its six-sided polyhedrons with diamond-shaped faces; the angles defining the faces are 78° and 102°.
The three important crystal habits (distinctive shapes of the mineral) of calcite are: (1) prismatic (both short and long), (2) rhombohedral, and (3) scalenohedral. Twinning is very common and may be of secondary origin in crystalline limestones. Some calcites fluoresce under ultraviolet light; some are also triboluminescent (luminescent when scratched). When light passes through some minerals, it is split into two rays that travel at different speeds and in different directions. This phenomenon is known as birefringence. The difference between the velocities is especially notable in calcite, and consequently crystals of a colourless variety—sometimes called Iceland spar—exhibit double refraction that can be observed with the naked eye.
The three important crystal habits (distinctive shapes of the mineral) of calcite are: (1) prismatic (both short and long), (2) rhombohedral, and (3) scalenohedral. Twinning is very common and may be of secondary origin in crystalline limestones. Some calcites fluoresce under ultraviolet light; some are also triboluminescent (luminescent when scratched). When light passes through some minerals, it is split into two rays that travel at different speeds and in different directions. This phenomenon is known as birefringence. The difference between the velocities is especially notable in calcite, and consequently crystals of a colourless variety—sometimes called Iceland spar—exhibit double refraction that can be observed with the naked eye.
