The Unit of Weight and Decimal System for Gems

The Unit of Weight for Precious Stones. The present unit for precious stones in the United States is the metric carat. Most of the more progressive countries have in recent years agreed upon the use of this unit. Its use in the United States became general July 1, 1913. It is by definition exactly one fifth of a gram (the unit of weight of the Metric System of weights and measures). Its relation to the grain is that there are 3.08+grains in the metric carat. The carat in use in this country up to a few years ago was about 2 1/2% heavier than the present metric carat. It was equal to .2053 grams instead of .2000 grams (1/5 gram). The carats of countries not using the metric carat vary considerably, but yet approximate the metric carat somewhat nearly.

Thus, that in use in Great Britain was .2053 g., in Amsterdam .2057 g., in Berlin .20544 g., in Lisbon .20575 g., and in Florence 0.1972 g. The latter was the only one that was under the metric carat. The change to the metric carat was desirable, as it unified the practice of weighing, which not only varied in different countries, but even in the same country. Thus there was no very exact agreement among the makers of diamond weights in the United States prior to the adoption of the metric carat. One man's carat was a bit heavier or lighter than another's. With a definite and simple relationship to the standard gram there is now no excuse for any variation in weights. The Bureau of Standards at Washington affords manufacturers every facility for standardizing their weights.

The Decimal System of Subdivision of the Carat. With the adoption of the metric carat the custom of expressing parts of a carat in common fractions whose denominators were powers of the number 2 (1/2, 1/4, 1/8, 1/16, 1/32, 1/64) was discarded as awkward and slow for computation and the decimal system of subdivision was adopted. Thus the metric carat is divided into tenths and one hundredths. It is customary, however, to sum up the one hundredths and express them as the total number of one hundredths and not to express them as tenths. Thus, a stone of 2.57 carats is said to weigh "two and fifty-seven hundredths carats." The decimal system of subdivision of the carat makes the figuring of values simpler where no tables are handy. Of course, new tables were at once prepared when the new carat was adopted and they afford a rapid means of ascertaining the value of a stone of any weight when the price per carat is known. Should it become necessary to convert the weight of a stone from its expression in the old system to that of the new, one need only get 1.02 1/2% of the old weight. (The old carat was approximately .205 g., while the new one is .200 g. Hence one old carat is .205/.200=.102 1/2/.100=102 1/2% of a new one.)

Method of Converting Weights. If the old weight has fractions these should first be changed to decimals for convenience. For example, suppose it is wished to change 2 1/4 1/16 old carats to metric carats. 1/4 = .25 and 1/16 = .0625. Hence 2 1/4 1/16 = 2.3125. Now get 102 1/2% of this: (2.3125 X 1.025 = 2.37 metric carats).

If, for any reason one should need to change from metric carats to old U.S. carats one should multiply by .9756

(.200 g./.205 g. = .9756)