A variant or archaic spelling of 'carat', a unit of weight for precious gems and gold purity.
From Arabic 'qīrāṭ', derived from the carob seed, which was once used as a standard unit of weight because of its uniform size. The spelling 'carrat' shows older English spelling conventions.
Medieval gem traders used carob seeds as a scale because they were so uniform in weight—eventually this became the 'carat' standard, and you can still see the carob heritage in words like 'carrot' and old spellings like 'carrat'.
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