Stable gemstones remain unchanged by heat, light, or chemicals. For example, strong light can cause yellow topaz to fade. Toughness refers to the brittleness of a gemstone. The more brittle it is, the easier it will break. Hardness, which is different from toughness, is the ability of a gemstone to resist scratching and general wear.
Diamonds are the hardest of all gemstones.
Gemstones usually go to a lapidary after they are mined. The lapidary does the final cleaning, which may involve grinding off the matrix, or rock, in which the gemstone was buried. While some gemstones may be simply polished, most are cut. The softer ones, like turquoise, are usually cut in rounded shapes called cabochons. Cabochons were the only way in which stones were cut until about the 14th century, when faceting was developed.