In order not to confuse karat with "carat", Americans changed the spelling. We obtuse Brits still habitually use the same spelling and word for two different and easy to mix up jewellery terms. Carat is a measure of the weight of gemstones, whereas karat is a measure of the purity of gold. The ratio of gold to other metals is measured in 24 parts, called karats - hence 24 karat gold is pure gold, while, for example, 9 karat gold is 9 parts gold and 15 parts other metal.