History of the Carat

from the Gemological Institute of America

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.

 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 relatively 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.

 Rules and Industry Practice

The FTC's Trade Practice Rules for the
Jewelry Industry (1957) state:

RULE 32: Misrepresentation of Weight, "Total Weight"  (a) It is an unfair trade practice to misrepresent the weight of any diamond or to deceive purchasers or prospective purchasers as to the weight of any diamond (Note: The standard unit for designation of the weight of a diamond  is the carat, which is equivalent to two hundred miligrams (1/5  gram). While advertisements may state the range of weights of a group of products, all weight representations regarding individual  products shall be subject to a 1/200th of a carat (one-half "point") tolerance.)

(b) It is an unfair trade practice to state or otherwise represent the weight of all diamonds contained in a ring or other article of jewelry unless such weight figure is accompanied with equal conspicuity by the words "total weight," or words of similar import, so as to indicate clearly that the weight shown is that of all stones in the article and not that of the center or largest stone.  Apparently, this guideline was inadvertently omitted from the  FTC Guides for the jewelry Industry (1979). It will probably be reinstated  in future revisions. The rule regarding representation of total weight is fairly plain: If you are showing a piece of jewelry set with more than one gem, be sure you are clear in your description of the weights of individual stones.

The "half-point" tolerance means that, in the US, weight must legally be measured to a thousandth of a carat (0.001)  and rounded to the nearest hundredth (0.01). It also means that a diamond weighing 0.995 carat can legally be described and sold as a one-carat diamond in the United States.

In many major diamond-trading countries and international industry organizations, a diamond's weight can only be rounded up to the next higher hundredth from nine thousandths of a carat.  Following this convention, a stone which weighs 1.768 carats  would be rounded to 1.76 carats; but one which weighs 1.769 carats would be rounded to 1.77 carats. Since many of the Quality Analysis Reports prepared by GIA's Gem Trade Laboratories are used internationally, the labs follow international practice.

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