Carats in diamonds refers to a weight measurement. It was based on the carob seed which has a very regular weight. The unit of carats to measure stones was officially introduced in 1907. There are 5 carats in 1 gram. i.e. 1ct = 0.2g. Confusingly gold is also referred to in carats. Americans us the ‘ct’ as a weight measurement and ‘k’ to indicate carats as a ratio in an alloy. In South Africa we generally use ‘ct’ to indicate both weight and a ratio of alloy. The typical ratios of gold used in South Africa are 9ct, 14ct and 18ct. 24ct is pure gold. When you refer to 9ct you refer to a ratio of 9/24ths (a minimum of 9 parts out of 24) are gold in the alloy. 9 divided by 24 as a percentage is 37.5%, which is the amount of gold in the mixture. Gold jewellery is hallmarked to show the amount of gold in the mixture. Either the carat or the amount of gold over 1000 is used as the hallmark. 9ct= 375, 14ct= 585, 18ct= 750.