Origins
The word 'carat' -- the unit by which diamonds and other precious stones are weighed worldwide -- owes its existence to the carob tree. Its etymology traces through a remarkable chain of transmission: from Greek 'kerátion' (κεράτιον, carob seed), through Arabic 'qīrāṭ' (قيراط), Italian 'carato,' and French 'carat,' before arriving in English in the 15th century.
The Greek starting point is 'kerátion,' the diminutive of 'kéras' (κέρας, horn), from PIE *ḱer- (horn, head). The carob pod was called a 'little horn' because of its curved, horn-like shape. The seeds within the pod became the focus of attention because ancient Mediterranean traders believed them to be remarkably consistent in weight -- each seed supposedly weighing almost exactly the same as every other. This perceived uniformity made carob seeds a natural choice as counterweights when weighing precious stones and gold on balance scales.
Modern botanical research has complicated this story. Studies have shown that carob seeds are not, in fact, significantly more uniform in weight than the seeds of many other plants. Their average weight is approximately 200 milligrams, but individual seeds can vary substantially. The reputation for uniformity may itself have been a self-fulfilling prophecy: traders who selected seeds carefully, discarding outliers, would have produced sets of reasonably consistent weights. Alternatively, the story may be a post-hoc explanation for a practice that originated for simpler reasons -- carob trees were ubiquitous across the Mediterranean, and their seeds were convenient, durable, and readily available.
Greek Origins
The transmission through Arabic is significant. The Arab world was the intermediary through which much ancient Greek knowledge and many Greek loanwords reached medieval Europe. Arabic 'qīrāṭ' was both a unit of weight and, in some traditions, a division of the gold dinar (1/24 of a dinar), which may have influenced the parallel use of 'carat' (spelled 'karat' in American English) to express the fineness of gold. Pure gold is 24 karats; 18-karat gold is 18/24 pure -- a system that may echo the Arabic monetary division.
The standardization of the carat as exactly 200 milligrams is surprisingly recent. For centuries, the carat varied from region to region, with the Florentine carat, the Venetian carat, the London carat, and others all differing slightly. It was not until 1907 that the Fourth General Conference on Weights and Measures established the metric carat of 200 milligrams as the international standard, a definition subsequently adopted by virtually every gem-trading nation.
The PIE root *ḱer- (horn) that lies at the base of 'carat' is one of the most productive roots in Indo-European languages. It produced Greek 'kéras' (horn) and 'kerátion' (carob seed > carat); Latin 'cornu' (horn), giving English 'corn' (on the foot), 'corner,' 'cornucopia' (horn of plenty), and 'cornet'; and it connects to 'keratin' (the horn-like protein of hair and nails), 'rhinoceros' (from Greek 'rhino-keros,' nose-horn), and 'unicorn' (from Latin 'uni-cornis,' one-horn). The fact that diamonds are weighed in units named after 'little horns' places the carat in an etymological family that includes African megafauna, mythical beasts, and the protein in human fingernails.
Modern Legacy
The English spelling 'carat' (for gemstone weight) versus 'karat' (for gold purity) is an American distinction not universally observed. British English uses 'carat' for both senses. The variant spellings entered English at different times and through slightly different channels, but both trace to the same Arabic 'qīrāṭ' and ultimately to the same Greek carob seed.