George Frederick Herbert Smith

UNIT OF WEIGHT

THE system in use for recording the weights of precious stones is peculiar to jewellery. The unit, which is known as the carat, bears no simple relation to any unit that has existed among European nations, and indubitably has been introduced from the East. When man in early days sought to record the weights of small objects, he made use of the most convenient seeds or grains which were easily obtainable and were at the same time nearly uniform in size. In Europe the smallest unit of weight was the barley grain. Similarly in the East the seeds of some leguminous tree were selected. Those of the locust-tree, Ceratonia siliqua, which is common in the countries bordering the Mediterranean, on the average weigh so nearly a carat that they almost certainly formed the original unit. It is, indeed, from the Greek ?e??t???, little horn, which refers to the shape of the pods, that the word carat is derived.

It is one of the eccentricities of the jewellery trade that precision should not have been given to the unit of weight. Not only does it vary at most of the trade centres in the world, but it is not even always constant at each centre. The difference is negligible in the case of single stones of ordinary size, but becomes a matter of serious importance when large stones, or parcels of small stones, are bought and sold, particularly when the stones are very costly. Attempts have been made at various times to secure a uniform standard, but as yet with only partial success. In 1871 the carat defined as the equivalent of 0·20500 gram was suggested at a meeting of the principal jewellers of Paris and London, and was eventually accepted in Paris, New York, Leipzig, and Borneo. It has, however, recently been recognized that in view of the gradual spread of the metric system of weights and measures the most satisfactory unit is the metric carat of one-fifth (0·2) gram. This has now been constituted the legal carat of France and Belgium, and no doubt other countries will follow their example. The carat weight obtaining in London weighs about 0·20530 gram, and the approximate equivalents in the gram at other centres are as follows:—Florence 0·19720, Madrid 0·20539, Berlin 0·20544, Amsterdam 0·20570, Lisbon 0·20575, Frankfort-on-Main 0·20577, Vienna 0·20613, Venice 0·20700, and Madras 0·20735. The gram itself is inconveniently large to serve as a unit for the generality of stones met with in ordinary jewellery.

The notation for expressing the sub-multiples of the carat forms another curious eccentricity. Fractions are used which are powers of the half: thus the half, the half of that, i.e. the quarter, and so on down to the sixty-fourth, and the weight of a stone is expressed by a series of fractions, e.g. 3½?¹/₆₄ carats. In the case of diamond a single unreduced fraction to the base 64 is substituted in place of the series of single fractions, and the weight of a stone is stated thus, 4⁴⁰/₆₄ carats. With the introduction of the metric carat the more convenient and rational decimal notation would, of course, be simultaneously adopted.

Figs. 34–39 illustrate the exact sizes of diamonds of certain weights, when cut as brilliants. The sizes of other stones depends upon their specific gravity, the weight varying as the volume multiplied by the specific gravity. Quartz, for instance, has a low specific gravity and would be perceptibly larger, weight for weight; zircon, on the other hand, would be smaller.

It has been found more convenient to select a smaller unit in the case of pearls, namely, the pearl-grain, four of which go to the carat.

Stencil gauges are in use for measuring approximately the weight in carats of diamond brilliants and of pearls, which in both instances must be unmounted. A more accurate method for determining the weight of diamonds has been devised by Charles Moe, which is applicable to either unmounted or mounted stones. By means of callipers, which read to three-tenths of a millimetre, the diameter and the depth of the stone are measured, and by reference to a table the corresponding weight is found; allowance is made for the varying fineness of the girdle, and, in the case of large stones, for the variation from a strictly circular section.

Since this chapter was written the movement in favour of the metric carat has made rapid progress, and this unit will soon have been adopted as the legal standard all over the world, even in countries, such as the British Isles and the United States, where the metric system is not in use. The advantage of an international unit is too obvious to need arguing.