Precious Stones are a type of gemstone, which are mineral crystals that are used for jewellery. They are often cut in a way that makes them look interesting, and they are polished to give them a shine. The exact definition of what gemstones are and aren't precious stones varies, though there are five types of gemstones known as the Cardinal gems which are considered the most precious, these are rubies, diamonds, sapphires, emeralds and amethysts, though there are some other stones that are sometimes considered precious stones such as pearls or opals. The generally accepted definition of a precious stone is a gemstone that is considered very valuable.