The Carat, a unit of measurement used in precious gemstones, was originally based on Carob Seeds.

Back in the day, people believed the seeds of a Carob Tree (a tree found in the Mediterranean) had uniform sized seeds. So they became the standard for weights.

But in actuality, the seeds did vary in size and weight. Unscrupulous merchants figured this out and had 2 baskets of seeds, one filled with heavier seeds for buying, and one with lighter seeds for selling.

Today, a carat equals 200 mg.

The term karat, used to signify purity in gold is related to the whole carat/carob thing. The weight of the gold being measured is compared to the weight of the same volume of pure gold.

The formula below is used to get the karat rating:

Mg is the weight of pure gold, and Mm is the weight of the sample being rated.

So 24 Karat Gold is almost totally pure. (Mg/Mm would equal a number very close to 1. Multiply times 24 for 24 karat.)

Source: History Channel and wiki for the assist.