There are a number of gemstones that are classified as collector's stones because they are too soft, too brittle, or too rare for the jewelry market. Calcite is a good example, though not because of its rarity.
Calcite is a carbonate mineral and is the most stable form of calcium carbonate, one of the most common minerals on earth. With a hardness of only 3 on the Mohs scale, it can only be used in jewelry that won't be subjected to knocks or scratches, such as earrings and pendants. Its hardness is similar to that of gems like amber, coral and chrysocolla. However, calcite is not only soft but, like most carbonates, it is senstive to acid, which can dissolve it.
Though calcite is not an especially durable material, it does take an excellent polish. Many historical artifacts identified as alabaster are in fact made from calcite.
Pure calcium carbonate is colorless, but calcite is often colored by various impurities, including iron, magnesium, manganese, zinc or cobalt. Thus calcite can be found in a range of colors including white, gray, yellow, green, red and blue.
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 all sedimentary rock is limestone.
Calcite is the primary mineral in metamorphic marble. It also occurs as a vein mineral in deposits from hot springs, and it occurs in caverns as stalactites and stalagmites.
Calcite is the primary ore of calcium. Calcite is indispensable in the construction industry, forming the base of cement. Many important chemicals are created from calcite. It is also crucial in the manufacture of fertilizer, metal, glass, rubber and paint. The transparent Iceland Spar variety, in which the double refraction is clearly apparent, was used as prisms for polarizing microscopes and other optical devices. Calcite forms rocks that are used for ornamental purposes, such as various kinds of marble. Calcite is also the main component of chalk.