Some famous gems are believed to carry a curse. Most gem stones are hard minerals. Their colours range from the colourless transparency of diamonds to pure black. Some are mottled or banded. Others reflect light waves in sparkling brilliance.
Gem minerals are usually crystal.
Mineral crystals vary in size from huge blocks weighing many tons to tiny specks that can be seen only under a microscope. Rock crystal, a common gem material, is found in chunks large enough to be carved into massive vases and plates. The crystals of chalcedony are so tiny that this mineral was long thought to be noncrystalline. A few gem materials are organic substances or have an animal or vegetable origin. The pearl, for example, is formed from secretions of molluscs.