(n.) a unit used to measure the quality and purity of gold
The carats that pure gold are measured in are related to rhinos.
Carat, or karat, has been on quite the etymological journey to get to where it is today. Adopted into English in the sixteenth century, it was initially borrowed from French—but French (likely with a little influence from Latin) in turn took it from Arabic, qirat, which in turn adopted it from the Greek keration.
As we explained over on Twitter, as well as being a unit of weight (once equal to about 4 grains, or 1/144 oz, although the precise figures have changed repeatedly over time), keration was the Greek word for the seed of the carob tree. Those seeds hang down from the tree in flattened and often coiled or curled tubes that apparently resemble the spiralling horns of goats and antelopes. Because of that shape, keration itself derives from the Greek word keras, meaning ‘animal horn’—which, via yet more etymological jiggery-pokery, is also the origin of rhinoceros.
This family tree of words doesn’t stop there, however. Keras is also the origin of keratin, the fibrous protein from which rhino horns (and human hair and fingernails) are made. It’s the reason why the triceratops’ name literally means ‘three-horned face’, and why its Jurassic relative the ceratosaurus was literally a ‘horned lizard’.
Journey even further back in time, and keras has its own roots in a Proto-Indo-European root, *ker–, that was likewise used to form words denoting heads and horns way back at end of the Neolithic period. From that ancient root, Latin too developed its own word for an animal’s horn, cornu, which has gone on to give us the words unicorn, cornet, cornucopia, Capricorn and even the cornea in your eye, which was so called for its horn-like structure.