This is the unit of weight used for diamonds, a word derived from carob seeds used to balance scales in ancient times.
A carat is equal to 200 milligrams and there are 142 carats to an ounce. Carats are further subdivided into points. There are 100 points to a carat. For example, a 45-point diamond weighs a little less than half a carat. Because larger diamonds are quite rare, they have a greater value per carat.