Weight and Specific Gravity

: Contrary to popular understanding, the fact that a gem weighs, say, ten carats has little to do with its size, except when compared with a gem, 1 carat diamonds or 2 carat diamonds of its own species and of similar cut. The carat is an expression of weight, not size. One carat is equal to one-fifth of a gram; in more familiar units, there are about 140 carats in an ounce. This standard unit of measurement of gem-stone weights was not legalized in the United States until 1913. Before 1900 the several weight units used throughout the world differed somewhat, but the carat in one form or another had been used since ancient times. Speculation relates the weight of the carat to the weight of a seed of the tropical carob tree. These seeds are unusually constant in weight and are just slightly lighter than our legal metric carat. The ancient Greek weight—the ceratium—and the Roman siliqua are both about the same as the carob seed. Most likely, then, the name and the weight did originate with the carob tree.

Even knowing that the carat is an expression of weight only, gem buyers are often surprised to see that a one-carat sapphire is considerably smaller than a one-carat diamond. Sapphire is denser than diamond and a smaller stone can weigh the same number of carats as a larger diamond. Since size and weight are both very important measures for gemstones, the unit of .measurement that combines both— "density"—is fundamental.

It is complicated to think of a gem's density, because size and weight must be considered simultaneously. We are aware of a difference in weight when we compare iron and wood. Yet it would not always be correct to say that iron weighs more than wood because a large piece of wood can weigh more than a small piece of iron. Only by comparing equal volumes of these materials can the extent of the weight difference be made clear and unmistakable. Making innumerable comparisons of the weights and volumes of different solids with each other seems inconvenient. To avoid this, it is customary to compare all of them with some selected substance, readily available, with a known density. Water is the standard most often used. Ruby, for example, proves to be four times heavier than an equal volume of water, so its specific gravity—the term of comparison with water—is 4. Diamond weighs SlA times as much as an equal volume of water and thus has a specific gravity of 3Vi.