The modern carat system for diamonds started with an assuming beginning: the carob seed. Early diamond traders used the small, uniform seeds as the counterweights in their balance scales.
The carob seed comes from the locust tree. The seeds grow in pods that are used for flavoring and for livestock feed.
The little carob seed was a pretty decent measuring unit, but over time more precise measurements were developed. It was only in the early 20th Century that a true weight of .20 grams was established to equal exactly one carat of diamonds.