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Chemical composition -- Calcium carbonate. The colorless transparent crystals are known as Iceland spar.

Color -- Colorless or shades of pink, purplish red (cobaltocalcite), green, blue, yellow.

Optics -- R.I 1.49-1.74. Extremely high birefringence (0.172-0.190). The extreme birefringence makes calcite useful in instruments such as the dichroscope, which displays the pleochroic colors of gemstones.

Durability -- Hardness 3. Difficult to facet due to perfect rhombohedral cleavage. Too soft for jewelry use. Massive forms are used for architectural and ornamental items, such as ashtrays, bookends, small sculptures, etc.

Crystal structure -- Hexagonal . Often occurs in massive forms (true onyx).

Petoskey stone is a greenish to grayish to brownish fossilized coral from Petoskey, Michigan. Cabochons of this material show a distinctive cell-like appearance.

Specific Gravity -- 2.71-2.94.

Sources -- Calcite is the most abundant carbonate material and is found throughout the world. Red or pink cobaltocalcite makes magnificent, scarce collector's items due to difficulties in faceting.

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