A gemstone or gem is a piece of mineral, which, in cut and polished form, is used to make jewelry or other adornments. However, certain rocks or organic materials that are not minerals, are also used for jewelry, and are therefore often considered to be gemstones as well. Most gemstones are hard, but some soft minerals are used in jewelry because of their luster or other physical properties that have aesthetic value. Rarity is another characteristic that lends value to a gemstone. Apart from jewelry, from earliest antiquity engraved gems and hardstone carvings, such as cups, were major luxury art forms. The carvings of Carl Fabergé are significant works in this tradition.