Edit: this is of course, pretty much exactly what was posted. I intended to paste this to my Facebook, and got mixed up. Oops.

Carob seeds were used in balance scales. Gold and gems, being precious, were weighed out in tiny quantities, and so small counterweights were needed.

>How did the carat system start?

>The modern carat system started with the carob seed. Early gem traders used the small, uniform seeds as counterweights in their balance scales. The carat is the same gram weight in every corner of the world.

https://4cs.gia.edu/interactive-4cs/carat-weight/origins.html