carat

the word 'carat' entered english from the french 'carat', which itself came from the italian 'carato'. this traces back to the arabic 'qīrāṭ', a small weight unit, and ultimately to the greek 'keration', meaning a small seed or weight. the greek term is a diminutive of 'keras', meaning 'horn', likely because the seeds of the carob tree, which were used as standard weights, resemble a small horn in shape. thus, the term's journey from a literal seed used for weighing to a precise unit for gemstones and gold purity illustrates a classic case of metonymy, where the name of a measuring tool becomes the name of the measurement itself.

the word 'carat' is a loanword, borrowed as a whole unit from french. it is not productively formed from smaller, meaningful english morphemes.