The cutting referred to here is the shaping of the facets and the surfaces of the stone; for most crystalline formations, however hard, are brittle; that is, they may be split or cleaved along the lines of the crystal edge. This accounts for both the possibility and the danger of cleaving the raw diamond. Formed under tremendous pressure beneath the surface of the earth, a diamond may be distorted in its growth; it may be an unshapely and often a fairly large stone, which must be cleft to a proper shape and size for setting. This cleaving, effected by a single hammer tap, is made only after minute examination and re-examination—sometimes a year’s pondering—by an anxious expert. After this cleavage, a diamond will be much smaller than when it was mined, but it counterbalances the loss of size by the greater brilliance and beauty the new shape discloses. The Kohinoor diamond was over 700 carats when it was found; when cut it was no more than 186⅙ carats; and since then it was recut, as a brilliant, to its present weight of 106⅙ carats. Sculpture has been defined as the process of removing the excess from the marble statue already within the block; how much more this is true of the precious stone, which the lapidary releases from its dull confinement!
Diamond dust, black diamonds, and hard metals may be used to shape, engrave and polish the other stones. A list of some of the stones frequently used in ornaments and jewels would rank them, for hardness, in the following order:
Qualities of a Stone
The qualities that determine the value of a stone are difficult to specify. Hardness, size, weight and shape are obvious elements. Lustre and the powers of reflecting and refracting light clearly contribute to the value. The manner in which a stone is cut may add to its value, either because of the light effects or because of the interesting shape. One might expect perfection, freedom from flaw, to be important, and indeed in the diamond this is so. The most common flaw in the diamond, by the way, is not a crack but a speck or tiny specks of carbon remaining between the crystals, the diamond being a crystallized form of carbon. In other stones—as we have observed of the star ruby, the star sapphire and the cat’s eye—a physical flaw may result in a greater aesthetic desirability. Other special features may enhance the value of a particular stone; a recently discovered ruby is the only known example of a double star, with not six but twelve rays. The history and associations of a gem or jewel, dramatic or sentimental, storied or personal, may be what makes its possession desirable.
Measurement
One seldom speaks of the size of a precious stone; other things being equal, its value is estimated by its weight. The unit of weight, in measuring precious stones, is the carat. As the word carat comes from the Arabic, meaning the nut or bean of the carob tree, it was evidently in olden times a rather imprecise measure. It has now been made definite as two-tenths of a gram (1c. = 0.2 gr.). It takes 141¾ carats to make an ounce, and therefore 2,268 carats to make a pound. Smaller diamonds are measured by points; one hundred points equal one carat.
The pearl is usually measured by the grain; a grain equals ¼ of a carat, or one twentieth of a gram (0.05 gr.). (This grain is not to be confused with the grain that is the smallest unit in the English system of weight.) Any pearl which is less than one quarter of a grain is called a seed pearl; an ounce of these may contain as many as 7,000 to 9,000 pearls. They are used in embroidery, in weaving cloth, and for many-stranded chains.