A gemstone (or gem, fine gem, jewel, precious stone, or semi-precious stone) is a piece of mineral crystal which, in cut and polished form, is used to make jewellery. Certain rocks, including lapis lazuli and opal) and some organic materials that are not minerals (such as amber, jet, and pearl) are also used for jewellery and are therefore often considered to be gemstones as well.

Most gemstones are hard, but some soft minerals are used in jewellery because of their sheen or other physical properties that have aesthetic value. Rarity is another characteristic that lends value to a gemstone.