The carat system sprouted from a carob seed.
The carob seed comes from the locust tree. The seeds grow in pods that are used for flavoring and for livestock feed. Because the small seeds are fairly uniform in size and weight, they provided a fairly consistent basis for gem weight. Early gem merchants and jewelers used them as counterweights in handheld balance scales.
Even when more precise weighing methods evolved, carat weight wasn’t constant: A “one-carat” diamond might have weighed anywhere from 0.95 to 1.07 metric carats. This was true until the early 20th century, when a carat was standardized as 0.20 gram. The carat now stands for the same gem weight in every corner of the world.