The processes through which a piece of carbon passes in the course of its conversion into a natural diamond are not known to man; all that is known is that the carbon crystallises. The natural home of the diamond is supposed to be a rocky matrix; but this is theory only, there being no evidence that a diamond has been discovered so situated. The mountains that supply the debris in which the gems are contained are composed of schistose rocks intermixed with quartz, sandstone, brescia, flinty slate, limestone, etc. All that is actually known concerning the original position of the diamond is that the only rock in which it is found in the mountains is the limestone brescia. Of course, as discovered, the rough gems are quite dull and lustreless, and it is necessary to cut and polish them; there must be principal planes or faces, and around these a considerable number of smaller ones placed at correct angles, so that by refraction a blaze of light, whose every ray is in harmony with the rest, may result. The facets must be so cut that light, in passing through, is refracted in such a way that a maximum of brilliancy is obtained. The diamond being the hardest of known substances, its own dust is the only available material with which it may be polished and worked.

The dust is obtained by grinding up worthless diamond particles in a steel mortar, the minute fragments obtained in the working of the diamonds also being saved for the purpose. Bort diamonds, black or grey carbon partly crystallised and found in conjunction with the ordinary diamond, are also used, these being jnst as hard as the clear diamond. Byr rubbing two diamonds together, they are mutually abraded or worn away; hence, a valuable diamond is cut by rubbing it with a comparatively worthless black one of equal hardness. Both clear and black or bort diamonds are fastened in the ends of sticks of cement, the black one having its cutting angle so placed that it may he used to the best advantage. The workman holds the clear stone in his left hand, and rubs it with the black one held in the other hand, the dust produced by the abrasion falling into a double metallic box whose inner receptacle lias a perforated bottom; the coarser particles left in this receptacle must be powdered in a steel mortar for use as a diamond polishing medium, whilst, the powder that has passed through the perforations is already sufficiently fine for the purpose. Diamond cutting is slow and tedious work, and requires the utmost care and skill.

It is possible to split or divide a diamond if advantage is taken of its grain or cleavage plane. Sometimes a large piece is split off a stone, and the time and expense of a prolonged abrading process avoided; but the splitting operation is a risky one. The stone is studied closely, and its line of cleavage is ascertained; it is then cemented to a suitable support. The sharp edge of a razor-like chisel is carefully adjusted to the line of cleavage, and a smart rap with the hammer is given to the chisel. If the splitting is successful, much expense will have been spared; if unsuccessful, it is Likely that the diamond is spoiled, and is comparatively worthless. In the polishing of the cut stones use is made of a turn-table. In Holland, which for many years monopolised the diamond cutting and polishing industry, ponderous machinery was employed, the wheels being braced and wedged like the running-gear of a sawmill. Since 1870 or so much interest in the industry lias been awakened in America, where a lighter, more compad and serviceable machine is in use. It is a small iron-top table having solid iron supports and double bearings, so that the polishing wheel in the centre revolves horizontally with its surface flush with that of the table; perfect steadiness is obtained with this machine.

The cut stone to be polished is fixed in soft lead heaped conically in a copper cup, and the flat surface of the wheel is charged with a paste of diamond dust and oil. The copper cup holding the diamond is placed in a heavy iron clamp, in which it is held inverted above the polishing wheel, which is then made to revolve at the rate of about 1,500 revolutions per minute. Nothing but the diamond presses upon the wheel, a rather musical sound being produced by the contact if the wheel is doing its work. When one facet has been polished, the wheel is stopped, the lead in the copper cup melted, the diamond reset, and another portion of the diamond is worked. Most careful measurements and experiments are necessary in polishing a diamond, so that accuracy of angular proportion in the facets is obtained.