The term ‘carat’ is used to measure the weight of gemstones and the fineness of gold. The origin of carat weight comes from the Sanskrit ‘quirrat’ and the Arabic qirat, describing the seed of the carob tree and its use as a weight for pearls by pearl-traders in antiquity. The word became incorporated into English in the Middle Ages from the Italian ‘carato’.
The carob seeds were thought to be uniform in weight, weighing about 0.197 grams, but in actual fact, the seeds do vary a bit in weight. This meant that there were varying standards for a carat weight used for gems in different countries and a number of attempts were made over the centuries to standardise the weight. At various times and in various countries, there were at least twenty-three standards for the carat ranging from 0.187g to 0.215g. Paris gem dealers attempted to standardize the carat weight as being 0.205g in 1871 but they couldn’t get international recognition.
Another attempt was made in 1907 by the Comité International des Poids et Mesures, setting the weight at 0.200g, a metric carat. This proposal had been supported by most European countries, as well as the Association of Societies for the Protection of Commerce in the UK, the Birmingham Jewellers & Silversmiths & Associations, and even the Association of Manufacturing Jewellers of the Colony in Victoria, Australia. Gradually, this weight became the accepted international standard. Australia adopted it on the 1 July 1913 and the UK adopted it on 14 October 1914. In the US, George Kuntz from Tiffany & Co strongly supported the introduction in the US of the metric carat. It was adopted in the United States on July 1, 1913 by the National Jewelers ‘ Board of Trade, the National Jewelers’ Association, the Bureau of Standards and the US Treasury Department.
So, is it carat or karat? The correct word when using it in the context of the weight of gemstones is carat.