In ancient times, just like today, many commodities were bought by weight. Only back then digital scales were not available and weighing small goods was quite a challenge.
To overcome the obstacle, ancient-times merchants used to scale gold and gems against carob tree seeds. These seeds were chosen because on the on-hand carob trees were prevalent in the Mediterranean and the Middle East, where most of mankind dwelled; and on the other hand, it was believed that all carob seeds weigh the same.
Karat is an abbreviation of carat (French) which derived from carato (Italian), which comes from Arabic qīrāṭ ( قيراط), which, in turn, was borrowed from Greek Keration keraiov (kerátion κεράτιον) meaning ‘carob seed’. The latter is a diminutive of keras meaning ‘horn’ as the Carob tree’s fruit has a horn shape.
Later, in Ancient Rome, gold was weighted in Libras. Nonetheless, to determine gold’s purity each libra was divided into 24 units – carato units. Solid gold was made out of 24 gold carato. 18 carato meant it was a blend of 18 units of gold with 6 units of another metal, usually copper or silver. It still does.