gem carbonates

Carbonates, one of the smaller groups of gem minerals, include malachite, azurite, and rhodochrosite, and the organic gem carbonates pearl, and coral. Malachite, the green basic carbonate of copper, is found associated with other ores of copper (especially azurite, the blue variety) in various parts of the United States and in Chile, Russia, Congo (Kinshasa), Zimbabwe, and Australia. Beautiful crystals of azurite are found in the United States in Arizona and New Mexico and in France at Chessy (for which the mineral is sometimes called chessylite). Gem quality rhodochrosite, a rose-colored manganese carbonate (MnCO3), ranges from opaque pink to transparent orange-red. Pearl consists of 92% calcium carbonate in the form of aragonite crystals, held together by 6% conchiolin, a horny organic substance, and 2% water. Hard, rounded secretions formed inside the shell of certain bivalve mollusks, the best pearls are usually white, sometimes with a creamy or pinkish tinge, but may be tinted with yellow, green, blue, brown, or black. Black pearls, because of their rarity, are often highly valued. The largest natural pearl center is the Persian Gulf, which is said to produce the finest saltwater pearls. Other important sources are the coasts of India, China, Japan, Australia, the Sulu Archipelago, various Pacific islands, Venezuela, and Central America, and the rivers of Europe and North America. In ancient times, the Red Sea was an important source. The Japanese, who have perfected the techniques of pearl cultivation, produce virtually all of the world's supply of cultured pearls . Gem quality coral—calcium carbonate in the form of calcite— is the massed skeletons of the small, sedentary marine animals of the same name. There are hundreds of species of coral; the color of the resultant gem material may range from white, pink, or red to black or blue.