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 κεράτιον,
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 ⅛ 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, 4-40/64 carats. With the introduction of the metric
carat the more convenient and rational decimal notation would, of
course, be simultaneously adopted.
[Illustration: Figs. 34-39.—Exact Sizes of Brilliants of various
Weights.]
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.