Carat usage for measuring weight
Later, with the almost universal introduction of the metric system of weights and measures, it was realized that a carat equivalent to .200, or one-fifth, of a gram would be far more convenient, and by the year 1913, all dealers had agreed to make this the universal carat. The general adoption has obviated the awkward use of fractions of carats, such as one-eighth and one-sixty-fourth (which were formerly employed), as well as the necessity for dealers to alter the weights of parcels of stones according to the market to which their goods were being dispatched. Also calculations were much simplified when prices or average weights were being considered.
Compared with ordinary weights in common use, a carat is small. A round diamond, proportionately cut and weighing one carat, would be the size of the illustration shown. Other gem stones would be slightly larger or smaller, according to how their specific gravities compare with that of diamond. This size, and others, can be memorized by the constant examination and handling o£ stones, and it is very useful for the dealer to keep such sizes in mind for the purposes of comparison.
When the carat is not used as the unit weight, Troy weights are used in England, and the metric system of grams and kilograms (1 kilo, equals 1,000 grams) in the U.S.A. and on the Continent. But the carat is used in connection with nearly all stones. With rough stones of small value, such as opal, amethyst, and turquoise, Troy ounces are used. Rough amber is bought by the kilogram or ounce Troy, coral by the gram, while such bulky material as rock crystal is bought in the rough by the hundredweight (avoirdupois) or kilogram.
The relative weights of the carat are 1 carat equals .200 gram, or one-fifth of a gram,
or, 3.08647 grains Troy,
or, .00705478 ounces Avoirdupois