A carat is a weight. A small weight. Very small... there are 150 to the ounce. Well, at least, there are these days. In days of yore there might have been 144 or even 24 carats to the ounce. Obviously, in those days it paid to keep abreast of the local conversion rates. Carat comes from the Greek keration "little horn" and refers to the fruit of the carob (Ceratonia siliqua) tree. Let us, who have seen their withered brown pods littering the sidewalks of Santa Clara, assure you of the accuracy of that description. Originally the seeds themselves were used as weights and their use was adopted by the Arabs who traded all over the Mediterranean. They pronounced keration as qirrat and it was this Arabic word which entered Italian as carato and French as carat.
Kutscher points out that the Jews also used carob seeds as a measurement of weight as well as a monetary value. He quotes Lazar ben Yose from the time of the early Arab conquest of Eretz Yisrael as writing that he paid with "27 haruvin." Ibn Ezra on Shmot 30:13, explains that weight gera גרה is referring to carob seeds (גרגרי חרוב).
Kutscher writes that from Greek, keration passed into Aramaic, and appears in Yerushalmi Peah 21a: חד קרט. He claims that the term entered Arabic via Aramaic.
Kutscher writes that there are those that say that keration has a Semitic origin, but he believes it is originally Greek. I assume he might be referring to the similarity between the Greek keras and the Hebrew קרן keren - both meaning horn. In any case, Klein does not buy into the widely accepted approach, and writes:
But the Greek word itself is a Semitic loan word; it is borrowed either from Aram.-Syr. קרטא (= pod, husk), or Arab. qaraz (pods of the acacia tree). Gk. keration in the above sense is a folk-etymological alteration of the Sem. word and has nothing in common with keration (= small horn), dimin. of keras.
Related words in Hebrew are koret קורט - meaning "a particle, a grain", and Almagor-Ramon connects kurtov קורטוב, a small liquid measure in Talmudic Hebrew, and today meaning "a pinch" (Klein does not make a connection between the two, and Jastrow says that koret comes from קרץ, pinch.)