Carat (K or kt) is a unit of measurement for the purity of gold. Whilst the term ’ounce‘ describes the mass of gold, ’carat‘ describes the proportion of gold in a metal alloy. The division for this is carried out in steps of 24, so for example 1-carat gold contains one 24th part of gold per total mass.
Since carat really defines the purity of gold, in practice the proportion of gold is often given in parts per 1000 which can easily be converted into a percentage. For example, the gold alloys 585, 750, and 875, which are common in jewellery, correspond to a gold content of 58.8%, 75.0%, and 87.5%. Since it is technically impossible to remove all impurities, 999 gold, i.e. a gold content of 99.9%, is regarded as the finest gold and corresponds to 24 carat.