In gemology, a gem is an ornamental object of high economic value, obtained from a natural mineral generally considered a precious mineral for its rarity and beauty. It can also have organic origin as in the case of pearls, corals and amber. The minerals used in gemology are numerous and some of them are used in more than one variety: thus, for example, ruby and sapphire are two varieties, red and blue respectively, of the same mineral, corundum. Emerald and aquamarine are two varieties of beryl. Most of the stones used as gems are in the crystalline state. In addition to gems of natural origin (real gems), artificial gems (synthetic gems) are also on the market.