A crystal is a structure in a solid state of aggregation that has a special composition. The elements forming it are not arranged randomly without a recognisable pattern, but follow a clear and regular structure. The crystal composition is therefore what makes a crystal a crystal. Examples of well-known crystals are sugar and salt, but also valuable minerals that are used in jewellery making as gemstones. The term crystal has its origin in Greek, where it means ice and frost. Basically, two types of crystals can be distinguished: Idiomorphic crystals and xenomorphic crystals. Idiomorphic crystals form their own, unaffected shape, while xenomorphic crystals allow their growth to be influenced by foreign interfaces. In the case of idiomorphic crystals, which can grow freely, the outer shape allows conclusions to be drawn about the crystal structure. This can be observed, for example, in cube-shaped crystals of the sodium chloride group such as common salt or halite. Of course, there are also deviations from the ideal shape in such naturally growing crystals. Crystals are known as gemstones in a wide variety of manifestations. The different coloured variations of quartz, for example, are popular gemstones and the diamond also belongs to the crystals, as it is the crystalline form of carbon.