All cutters have been compelled to follow this method, and the perfectly cut brilliant of today has a depth from table to culet of six-tenths of the diameter, of which one-third is above the girdle and two-thirds below. In this form the diamond resembles two cones united at their bases, the upper one cut off a short distance from its base, the lower one having its extreme point cut off. It has fifty-eight facets, of which thirty-three, including the table, are above the girdle and twenty-five, including the culet, below the girdle. Stones which are not scientifically cut in this true proportion, if too deep, are called “lumpy,” if too shallow they are called “fish eyes.” A slightly spread stone is desirable, provided it has not lost brilliancy, and so become a “fish eye.” Looking larger than its weight indicates, it offers a larger appearing diamond for the price of a smaller perfectly cut stone. Most cutters remove as little of the rough stone as possible in cutting so as to retain weight (they sell by weight). This often results in the finished diamond being too thick at the girdle, making a lumpy stone. Many people think deep, lumpy stones are most desirable. This is not true, as they are imperfectly cut.
In preparing to cut a diamond the rough crystal is studied until the grain is found. Along the grain another sharp-pointed diamond is ground until there is a V-shape incision or nick. The blunt end of a flat piece of steel is placed in this nick and a smart blow of a hammer divides the crystal evenly and perfectly. After this “cleavage” has removed the unnecessary portions, or they have been sawed off by the use of rapidly-revolving thin wheels charged with diamond dust, the diamond is set in a turning wheel and ground with another diamond until it takes the shape in which we know it.
The fifty-eight facets are cut and polished one at a time on a rapidly-revolving wheel charged with diamond dust and oil. It takes from two and one-half to four days to properly cut a stone. Knife-edge girdle diamonds are impractical owing to the liability of chipping the thin edge in setting or by blows while being worn. Polishing the rough edge of the girdle is rarely done and then usually to conceal a girdle which is too thick or lumpy. The principal diamond cutting centers are Amsterdam, Antwerp and New York.
Inherent flaws can be perfectly understood by imagining a pond of water frozen solidly to its center. At the shore, where the ice has been partly forced out along the banks, it will be full of grass, leaves, pebbles and sticks, and presents a broken and frosted appearance. Further out there are only traces of such débris, some bubbles, spots, etc. Out at the center is clear, transparent, unbroken, unflawed, purest blue-white ice, such as you delight to see in your glass on a hot day. So is it with diamonds; some (like the ice along the shore) are full of cracks, carbon specks, bubbles, clouds, splits and cavities; some have all of these; some only a few; others only one, and some are without flaws.
Of all the imperfections (not considering glaring cracks or nicks), carbon spots are the most discernible. They range from mere specks scarcely visible with a powerful magnifying glass, to large black spots or clusters of large or small black specks sometimes quite plain to the naked eye. These are carbon which failed to crystallize with the rest of the diamond, or intrusions of titanic iron. The blackest and often most numerous carbon specks occur in the finest white and blue-white stones. “Capes” and other yellow diamonds are usually perfect, something in the color of these stones seemingly being of a nature which helps clear and perfect crystallization. Blue-white stones of exceptionally fine color are often massed full of shaggy or jet-black carbon spots.
White specks and bubbles are common flaws, which vary in size and which may be best illustrated by looking at a pane of glass in your window. There you will find small knots, white bubbles and whitish specks. These seldom injure the brilliancy, as they are often a glittering silver color, more brilliant than the diamond.
Clouds are dark flat patches in the grain, of a brownish color, and appear as a sprinkling of dust in a small patch in the interior. This seldom injures brilliancy.
Glessen or glasses are flat sectional streaks having an icy appearance. When large or abundant they disturb or cut off the proper reflection of the interior light rays, causing an appearance known as “shivery.” When clouds or glessen occur at the surface of a diamond they appear as cracks, and if at or near the girdle are dangerous, as the stone is liable to split or crack there when being mounted or by any hard blow, which would result in the loss of a sliver or wedged-shaped piece out of the edge.