Generally speaking, a gemstone is a stone that is beautiful, rare, and durable (resistant to abrasion, fracturing and chemical reactions). Some minerals can be very beautiful, but they may be too soft and will scratch easily. Others are too common and are given a semi-precious status. Most gemstones have good hardness (above 5) and a high index of refraction (the higher the index of refraction the greater the sparkle). All gemstones have some characteristics falling short of perfection though; even the seemingly perfect Diamond. Most gems are silicates which can be very stable, hard minerals. A few gems are oxides and only one gem, diamond, is composed of a single element, carbon. There are also many gemstones that are not true minerals, but which are gemstone varieties of recognized minerals.