Carat -- Weighing Pearls

A carat is the common weight used as one of the determining factors in the value of natural pearls.

Woman Weighing Gold
Vermeer, Jan
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Yes, natural pearls are still being found and sold today. For example, one of my website visitors found a quahog pearl a couple of weeks her mouth!... as she was eating raw little neck clams. She contacted me wanting to sell it. I found a buyer and in a few days she was $2,700 richer! Not a bad return for $3 worth of fresh clams.

I have only seen pearls sold in a store by weight in Istanbul, Turkey. It surprised me when I asked how much the strand of big pearls cost that the jeweler put them on a scale, then gave me the price. Looking back, I'm thinking, maybe they were natural pearls. It didn't occur to me to ask if they were naturals....and if that is the case, I should have snatched them up for the good price.

The following information about the development of a standard carat weight is from pearl experts, Kunz & Stevenson, and first published in 1908. Keep in mind the age of this is about 100 years old, and precise details may differ today. I have sometimes abbreviated the word "carat" on this page to "c."

Diamonds and the more valuable precious stones generally are bought and sold by the weight called a carat. This c., whatever its precise value, is always considered as divisible into four diamond or pearl grains, but the subdivisions of the carat are usually expressed by the vulgar fractions, 1/4, 1/8, 1/12, 1/16, 1/24, 1/32 and 1/64.

The origin of the c. is to be sought in certain small, hard, leguminous seeds, which when dried, remain constant in weight. The seed of the locust tree, Ceratonia siliqua, weighs on the average three and one sixth grains, and constitutes, no doubt, the true origin of the c.

The following derivation of the word c. is given by Grimm: "Cartat. Italian: carato; French: carat; Spanish and Portuguese: quilate; Old Portuguese: quirate, from Arabic: qirat."

The c. is not absolutely of the same value in all countries. However, an agreement was arrived at, as the result of a conference between the diamond merchants of London, Paris, and Amsterdam, by which the uniform weight of a diamond c. was fixed at .205 of a gram, making the pearl grain .05125 of a gram. The standard, which was suggested in 1871, by a syndicate of Parisian jewelers, goldsmiths, and others dealing in precious stones, was subsequently (1877) confirmed. But there is still a lack of uniformity in the standard by which diamonds and pearls are bought and sold, and very serious discrepancies exist in the sets of c. weights turned out by different makers, although the international c. is almost universally used.

At the International Congress of Weights and Measures held at the World's Fair at Chicago in 1893, the writer suggested that the c. should consist of 200 milligrams, so that 1/2 of a carat would be 100 milligrams and 1/4 of a grain would be 12.5 milligrams. This would mean 5 carats or 20 grains to a French gram, and 5000 carats or 20,000 pearl grains to a French kilogram. This would depreciate the present diamond c. or pearl grain only about one per cent, and it would do away with the needless series of c. and grains of the many nationalities. It could be simply explained to any private individual in any country, especially as there are only two countries which do not use the metric system.

This c. has been earnestly endorsed, its introduction advocated, and its merits clearly shown, by M. Guilliame, of the French Bureau odes Arts et Metiers, whose energetic work has found a reasonable co-operation, in this country as well as in Europe, in introducing what will be a scientific, logical, comprehensive, and possibly the final and international c.; and any ancient, obsolete, or foreign c. can be readily reduced to this c. once the metric value of the former is computed.

In view of the difficulty of inducing the abolition of the c. in different countries, the German Federation of Jewelers decided to petition the imperial government for authority to use the c., in order that it might be legally recognized. Such a proposition not being in accord with the German laws in force on the subject of the metric system, it was proposed to substitute for the c. then in use one c. only, weighing 200 milligrams. This proposal was very favorably received in trade circles and may be taken into consideration by the International Committee of Weights and Measures. The Commission des Instruments et Travaux, to which this proposition was referred, recommended its adoption to the committee in the following terms:

"The Commission recognizes that it would be very desirable that the unit of weight of precious stones (the carat) which varies in different countries, should be made uniform, and should be reduced to the nearest metric equivalent. The weight of 200 mg., which is very close to the c. most in use (205.5 mg.), would seem to be the best for this purpose. The Commission believes that there can be no objection to this standard of 200 mg. being called 'the metric carat' in order to facilitate the abolition of the old carat."

This proposition, adopted at the meeting of the International Committee on the 13th of April, was communicated to the more important associations. The Chambre Syndicale de la Bijouterie, Joaillerie et Orfevrerie de Paris, and the Chambre Syndicale des Negociants en Diamants, Perles, Pierres Precieuses et des lapidaires de Paris assured the committee of their support of this measure.

The following is the text of the resolution which was passed by both the above associations in January, 1906:

"The Council, recognizing the advantages which would result to the international trade in precious stones from the use of a unit based on the metric system, desires that the metric carat of 200 mg. be universally adopted."

The German Federation of Jewelers passed the following resolution in August, 1906:

"The German Federation considers that it is both necessary and advantageous to replace the old carat by the metric c. of 200 mg.; it authorizes its president to approach the imperial government and the International Bureau of Weights and Measures, and the foreign associations in order that the metric carat may be introduced as soon as possible in all countries."

The Chamber of Commerce of Antwerp proposed, in a letter dated teh 7th of December, 1906, to rescind a decision of 29th of April, 1895, approving the adoption of a c. of 205.3 mg., when the metric c. of 200 mg. should come into universal use in the markets.

The Association of Jewelers and Goldsmiths of Prague formally authorized the German Federation to act in its name, in order that the reform should come about as soon as possible by international agreement, and the Association of Goldsmiths of Copenhagen has declared its willingness to support the reform. The Committee of Weights and Measures in Belgium prepared a law for the adoption of the metric c. in December, 1906.

Mr. Larking, president of the Chamber of Commerce of Melbourne, Australia, has transmitted by letter of September 16,1907, the following resolution of the Association of Manufacturing Jewelers of the Colony of Victoria:

"It is desirable that the c. weight should be the same in all countries, and our association approves a metric c. of 200 milligrams."

On October 16, 1907, the Association of Societies for the Protection of Commerce in the United Kingdom passed the following resolution:

"The Committee of the Association approves the attempt to urge the adoption in all countries of an international c. of 200 milligrams, and hopes that, in the interest of the unification of weights, it will prove successful."

The fourth General Conference fo Weights and Measures, held in Paris in October, 1907, passed this resolution:

"The Conference approves the proposition of the International Committee and declares that it sees no infringement of the integrity of the metric system in the adoption of the appellation 'metric carat' to designate a weight of 200 milligrams for the commerce in diamonds, pearls, and precious stones."

The following resolution was passed by the Birmingham Jewelers' and Silversmiths' Association, January 23, 1908: "That the best thanks of this Committee by conveyed to the Decimal Association for the good work they are doing, and this Committee expresses the hope that all countries will adopt an International Carat of 200 milligrams in weight." Finally, on March 11, 1908, the metric c. of 200 milligrams was adopted in Spain as the official carat for diamonds, pearls, and precious stones.

Pearls have become of so much importance to so many dealers that a special form of weight has been proposed for them. This would have a diamond form and not a square form, and it would be stamped "Grain" instead of "Carat." Another set would be stamped in milligrams, the regular milligrams weight with the pearl fraction above it, and they could even be made round so as better to designate the pearl.

The great value of pearls has suggested the making of a gage, called the Kunz gage, by means of which round pearls can be very accurately measured. Pearls of a given weight and perfectly spherical form have been weighed and then measured by this gage, and the theoretical diameters as computed from the measurement of a single pearl are in the majority of instances in exact accord with these actual measurements, the occasional variations in the smaller pearls barely exceeding the thousandth part of an inch. These discrepancies may be due to imperceptible divergences in sphericity or, possibly, to trifling difference in specific gravity.

The new and finer analytical balances weigh to the tenth part of a milligram, the two thousandth part of a carat, the five hundredth part of a grain; but this is not necessary. If the 200-milligram carat were used, the two hundredth part of a carat could readily be ascertained, and then a short-beam, rapid-weighing balance would answer every purpose and save much time for the dealer who must make many weighing in the course of a day. In an office where thousands of weighings were made in a month, the task was accomplished with such minute accuracy that the margin of error did not exceed one carat during this time.

There have been at all times men who possessed a delicate touch or a fine sense of feeling, but probably few men are living today who would be able to accomplish the feat attributed to Julius Caesar, namely, that of estimating the weight of a pearl by simply holding it in his hand. There are very few who can tell the weight of a pearl in this way, and while the story may be historically interesting, it is rather dubious.

Learn more about pearl characteristics after enjoying the history of standardizing the carat weight.

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