In the UK, karat with a k is used to refer to the fineness of gold (while carat with a c is diamond weight). 24 karat gold is considered pure gold, every karat below 24 is referred to by its percentage fineness in comparison to 24k gold. So 18k gold is 18/24, 75% pure, or 750 fine. NB sometimes, confusingly, the terms karat and carat are used interchangeably.