Diamond Carat Weight

History of The Carat
Weighing commodities as small and precious as gems demands a very small, uniform unit of weight. To meet this need, early gem traders turned to plant seeds that were reasonably uniform in size and weight. Two of the oldest were wheat grains and carob seeds. Both were common in the gem-producing and trading areas of the ancient world. Wheat was a dietary staple, and indidual wheat grains provided a plentiful and relatively uniform weight standard.Our modern pearl grain, troy grain, and avoirdupois and apothecaries' grains all derived from the wheat grain. (Diamond weights are sometimes approximated in grains) The carob, or locust tree, produces edible seed pods that are still important as feed for livestock and as a flavoring. Traders used the inedible seeds as a standard weight from which our modern metric carat evolved.

Carat weight was standardized in the early twentieth century. If you had purchased a 'one-carat' diamond in 1895, it might have weighed anywhere from 0.95 to 1.07 metric carats, depending on where you bought it. But between 1908 and 1930, the standard metric carat was adopted throughout most of Europe and in Japan, Mexico, South Africa, Thailand, the USA, and the USSR.

Consumers sometimes confuse the terms carat and karat. Although in some countries the two are synonymous, in the US, karat refers to the fineness of gold alloys (pure gold is 24 karat; 14 karat is 14 parts gold and 10 parts other metal or metals) and carat refers to gem weights.
-Gemological Institute of America

CARAT WEIGHT
When you tell people one diamond weighs more than another, they usually understand what you mean-but few consumers realize how precisely diamonds are weighed. Like most gems, diamonds are weighed in metric carats; one carat equals 0.2 gram-a little more than 0.007 (seven thousandths) ounce avoirdupois.In other words, it takes almost 142 carats to equal 1 ounce. But even this is not precise enough for something so precious. Even with rela- tively inexpensive diamonds, fractions of a carat can represent hundreds of dollars (thousands, with top-quality stones). For this reason, in the diamond industry, weight is measured to a thousandth of a carat and rounded to the nearest hundredth (or point).
To visualize how precise this is, consider that a point-a hundredth of a carat-is less than one fourteen-thousandth of an ounce. The term point can confuse consumers, who may think you are referring to the number of facets on the stone, or to the decimal point specifying hundredths.

-Gemological Institute of America