DIAMOND CUTTING.
Diamond cutting has changed little over the centuries. The basic principle of 'diamonds cut diamonds' still stands, with the introduction of machines to perform a part of the operation being only innovative aspect. The most important is the expertise of the individual that matters. Before a stone is cut, it is examined for its cleavage plane and any flaws. Like timber, diamonds have a grain: they may be cleaved along the grain or sawn against it. In cleaving, a nick is made with another diamond and, after mounting the diamond to be cut in a holder, a heavy steel blade is placed in the nick: one sharp blow is usually sufficient to split the stone cleanly. Sawing is carried out by a thin disc of phosphor-bronze coated with oil and diamond dust, which revolves at high speed and cuts through the diamond. It takes many hours to saw through even a small stone.
The shape of the rough stone will decide the cut of the polished gem. If the diamond is to fashioned into a brilliant, the most popular cut, its circular shape is achieved by 'bruting': it is fixed in a lathe and held against another diamond as it rotates, thus rounding off the corners and edges. The final process is polishing the diamond's facets. The polishing tool is a round plate of porous cast iron with a steel spindle running through it. The surface is coated with a mixture of olive oil and diamond dust. The plate is spun at high speed - 2500 revolutions per minute. The diamond is fixed in a holder at the precise angle required to obtain a perfect facet; regular inspection is essential to ensure the facet is being shaped correctly. Then the facet is poliished on a specially prepared part of the plate: this eliminates the tiny lines and irregularities caused by grinding and imparts a mirror-like surface to the gem. On completion of the facet the polisher will turn the diamond to the position of the next facet and repeat the process until all the facets have been polished. Mathematical percision for the angles of the facets in relation to each other is required in order to achieve maximum brilliance. A brilliant will have 58 facets, 33 on top and 25 underneath. Other polished shapes, although varying in the number of facets, have numbers standard to type. They are all polished similarly and with the same tools.
During the course of the various processes employed in cutting a diamond a very high degree of skill is necessary. This will ensure the completion of a polished gem which as a brilliance unequalled by that of any other gemstone.
A NOTE ON THE CARAT
Before the discovery of diamonds in South Africa the weight of the carat was somewhat ill-defined, varying in different countries. Around 1871 a Frenchman recommended that the carat weight should be standardized at 0.2053 grammes. At the time this recommendation was internationally accepted. Then in 1914, one supposes for convenience, the United States of America. Great Britain and other European countries agreed to introduce the metric carat, weighing 0.2000 grammes. Nine years later South Africa followed suit. The metric carat is, it will be observed, lighter than the old carat in the ratio of 2000:2053.
It follows that books published before the year 1914 will have recorded the weights of diamonds in old carats. However, since that date certain publications have maintained this practice and chosen to ignore the metric carat which is universally employed in the diamond industry today. In order to clarify the situation, where the weight of a diamond in metric carats is known for sure, this is the weight given in the text. In other instances the weights will be recorded in old carats together with the calculated weights in metric carats.