The word "carat" comes from the carob bean which is the fruit of the Ceratonia siliqua tree (native to the Mediterranean and Middle East). These beans were selected because each one weighs exactly the same, 0.2 grams. 142 of these beans make up an ounce. I suppose they can't really be "beans" in the leguminous sense, but let's not split hairs.
Now ya know, "and knowing is half the battle. GI-JOE!"