Gemstones and crystals are pieces of minerals that divide into precious and semi-precious stones. Broadly speaking, diamond, ruby, sapphire and emerald are considered 'precious' while all others are semi-precious. Gemologists identify stones by their chemical composition, their crystal structure and their habit which refers to the location where the stone is usually found. Gemstones are also classified into groups, types and varieties. Further classifications include light properties, hardness, fracture, colour and lustre. Material or flaws within a stone are called inclusions. The value of gemstones varies and can fluctuate greatly. Gemstones are often artifically treated to enhance their colour or clarity. Synthetic gemstones can be physically, optically and chemically identical to natural stones but are created in controlled laboratory conditions. Their colours tend to be more vivid and impurities are not present. Different cultures have developed traditions of crystal healing. Healers believe in the esoteric powers of gemstones. They select stones by colour and their metaphysical qualities. Wearing a gemstone as an amulet may be beneficial to physical and mental well being. This Guide presents 72 gemstones and includes physical and esoteric characteristics.