The Carat

(via The History and Use of Diamond:1962) S Tolansky writes:

In the Middle Ages and the period of the Renaissance the jewel which was really extensively worn, naturally enough by ladies, was not the diamond but the pearl. Pearls have been taken from the Persian Gulf since the days of the Macedonians, if not earlier, and from the time of the Ptolemies pearl fishing has been actively conducted in the Red Sea. It is well-known that certain freshwater mussels can produce excellent river pearls and this, too, has been a source of pearls since ancient times. The river pearls of Britain are even mentioned by Tacitus and by Pliny, and a breast-plate studded with British river pearls was dedicated by Julius Ceasar to Venus Genetrix. It is perhaps worthy of mention that in 1355 Scottish river pearls are referred to in a legal statute, and that in the time of Charles II the Scottish river pearl trade was on a scale big enough to justify a Parliamentary discussion.

However, it was essentially the pearls of the Orient which really flooded Europe and they were certainly used in great profusion by the great ladies at the courts. Pearl collars, pearl hair decorations and pearl girdles are seen very frequently in contemporary portraits. This use of pearls mounted to a climax at the time of Queen Elizabeth I. The well-known portraits of this Queen, now in the National Portrait Gallery, London, show her to be wearing thousands of pearls. In one painting an enormous pearl rope of several turns contained perhaps a thousand pearls, and the whole grand dress is simply covered with this gem. The weight must have been considerable.

Now it was from this extensive pearl trade of the Orient that the weight measure still used today for all gems, diamond included, arose originally. In early times, the smallest units of weight adopted in commerce were, generally, specified seeds. For example, the barley grain was the lowest weight unit in this country and there are still, by definition 7000 grains to the pound weight. Of course, such a weight is an average. The astute ancient pearl dealers of the Orient made the very striking discovery that the dry seeds of the fruit of the locust-pod tree are remarkably uniform in weight. This tree is very widespread over the Mediterranean and in the whole Near and Far East. The locust-pd tree produces a curved fruit, something resembling a flattened, hard banana in appearance, rather like a horn. The pearl traders noticed that no matter how old the tree, and no matter which part of the pod the seeds came from, the weights of the dry seeds were remarkably uniform. This seed being readily available, it was adopted by the pearl dealers as the unit of weight for measuring pearls. The old Greek name for the plant is Keration, from whence we derive the word carat. The carat was originally the weight of the seed of this locust-pod fruit.

That the seeds have an amazing uniformity in weight can be demonstrated today with precision balances. The small differences found, say less than 1/1000th part of an ounce, are too small to have been measurable by the simple balances available to the ancient traders. As far as they could measure, such seeds were absolutely identical, and this fact is a remarkable tribute to the astuteness of the ancient pearl traders.

For many centuries the adopted working carat was a little different in weight in different parts of Europe, but always more or less the same, i.e something near to a fifth part of a gram, or 1/140th part of an ounce. Strangely enough, although the carat has been in use in this country since Norman times at least, it was for long only an agreed measure of commerce and had no legal standing at all. Even up to the middle of the nineteenth century the persistent differences in different trading centers were still appreciable. The weight in milligrams of the carat was 197 in Florence, 205 in Berlin, 206 in Vienna, 207 in Madras. The values used actually ranged from 188 to 213 milligrams, a variation of some thirteen percent. This was too much to be tolerated for reliable international Committee on Weights and Measures in Paris proporsed the adoption of what is now called the metric carat, which was to be exactly 200 milligrams, i.e 1/5th of a gram, or 1/142nd of an ounce. The French Government sought international agreement, but this very sensible proposal was resisted by many jewelers. However, pressure was exerted, and in 1915 the Board of Trade in this country at last legalized the metric carat. This was carried out more or less simultaneously in Britain, Holland, Belgium and the U.S.A, the four countries in which the dealing in diamonds was mainly concentrated. Strangely enough, South Africa, a principal source of diamonds, only adopted the metric carat as late as 1923. It is now universally used for gemstones and this fact must be remembered when the weights of old historical diamonds are being discussed.

The carat is quite a small measure, yet so costly is the diamond that the carat is subdivided still further, especially in the United States, where it is divided into one hundred points. Thus a stone weighing one and a quarter carats is written 1.25 carats and this particular diamond would be described as being of weight one carat and twenty five points. The carat is still retained as of old for the pearl, though in this trade a coarser sub-division is usual, the pearl grain, which is a quarter of a carat.

It is not easy to visualize carat sizes, but the following will help. A natural octahedron of height about 1/10th of an inch could weigh 1/10th of a carat. An octahedron of height one quarter of an inch could weigh about 1 carat. For an octahedron of height half an inch, the weight might be 8 carats. The polished gem of the shape called the brilliant grades differently because of the material lost in shaping. A brilliant 1/10th of an inch across weights about 1/16th of a carat, while a 1 carat brilliant-cut gem has a diameter of about one quarter of an inch. A brilliant gem of diameter half an inch would weigh about 8 carats. It will be seen that a 1 carat gem is a sizeable stone, hence the need for the division into points when estimating prices, especially of the finished products. Since a good quality 1 carat, finished, brilliant shaped diamond can cost currently some $1600 (?) in the U.S.A that very small additional quantity of weight which is called the point, although no more than a mere 10 milligrams, i.e just about 1/14000th part of an ounce, is worth as much as $16 (?). It is not surprising that finished brilliants are very carefully weighed to 1/100th part of a carat nowadays by jewelers when valuation takes place.

As a reminder, attention is drawn to the unfortunate use of the word carat in this country to denote the fineness of gold. Pure gold is classed as 24 carat gold, and alloys are graded accordingly. For instance, 18 carat gold means that the alloy contains 18/24ths of pure gold, i.e. 75 percent American, and continental practice favors a different spelling for the gold carat, replacing the c by k to read karat. This is an admirable suggestion and we would be well advised to adopt this practice here to avoid confusion with the carat which internationally accpeted now as a gem weight and should have nothing to do with the alloy/gold content.