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Derivation of carat The word carat is derived from the Greekkerátion (κεράτιoν), “fruit of the carob”, via Arabicqīrāṭ (قيراط) and Italiancarato. Carob seeds were used as weights on precision scales because of their reputation for having a uniform weight. (However, a 2006 study by Lindsay Turnbull and others found this to not be the case – carob seeds have as much variation in their weights as other seeds.[2]) This was not the only reason. It is said that in order to keep regional buyers and sellers of gold honest, a potential customer could retrieve their own carob seeds on their way to the market, to check the tolerances of the seeds used by the merchant. If this precaution was not taken, the potential customer would be at the mercy of "2 sets of carob seeds". One set of "heavier" carob seeds would be used when buying from a customer (making the seller's gold appear to be less). Another, lighter set of carob seeds would be used when the merchant wanted to sell to a customer.In the distant past, different countries each had their own carat, roughly equivalent to a carob seed. In the mid-16th century, the Karat was adopted as a measure of gold purity, roughly equivalent to the Roman siliqua (
圖片參考：http://upload.wikimedia.org/math/9/4/0/9404d795a2f348ccae78cfc14988b6bb.png
of a golden solidus of Constantine; but was likely never used to measure the weight for gold.