DIAMOND, this remarkable and most useful gem has been considered at some length in the first volume, pages 175 - 180 - first as regards the processes of splitting, cutting, and polishing diamonds for jewellery, - then its use in the hands of the glass-cutter and glazier - and lastly several of the uses of the diamond as tools, and which applications include the formation of the jewelled holes of ruby and sapphire for watches and chronometers, every process of which requires the intervention of the diamond.
In this place it is proposed to describe the three different modes in which the diamond powder is prepared for the use of various artizans, as the subsequent chapters will treat of its practical application by the lapidary, gem-engraver, and others. 1. - Diamond Powder fob Lapidaries' Use. - Lapidaries generally purchase small imperfect diamonds, and the fragments removed by splitting or cleavage, in preparing stones for jewellery. These fragments are crushed in a hardened steel mortar with a cylindrical hole about half an inch diameter, and nearly two inches deep, the bottom of the cavity is hemispherical or constitutes perhaps the third part only of the circle, the pestle almost fits the aperture of the mortar and is curved to the same degree, there is also a cover that fits a recess in the mortar to prevent the escape of any of the valuable dust.
The pestle is struck a few blows with a light hammer, and is twisted round between each blow, this readily crushes the diamond, which, although so incomparably hard, is brittle from its crystalline structure. The fragments are carefully collected, and mixed with a little of the oil of brick, in a small cup or any convenient vessel, which should have a cover to keep the prepared diamond from being wasted. When not wanted for immediate use, the prepared diamond is kept in a pasty condition between two very small watch glasses, cemented with soft wax around their edges. 2. - Diamond Powder for Seal Engravers. - This is required to be much more finely pulverized than for lapidary work, therefore having been crushed as above, the fragments are ground into a thick paste, with a few drops of olive oil, in another pestle and mortar of hardened steel, the surfaces of which are both exactly spherical with a curvature of from one to two inches radius; this mortar has a tin cover that it may serve as the recipient for the powder which has been ground. Sometimes for reducing the powder after it has been crushed, flat grinders of hardened steel are employed, but these are less generally used than the spherical form. Rough diamonds of a dark steely colour are generally selected by the seal engravers, as these are considered the hardest stones.
3 - Diamond Powder for Watch Jewellers. - These artizans who use much larger quantities of diamond powder than the above, for cutting as well as for polishing rubies, sapphires, and topazes, pursue a different method. They purchase the fine dust, or diamond bort, that is rubbed off stones used for jewellery in the act of cutting them into facets, in which process two diamonds are operated upon at once, and caused mutually to abrade each other in forming the one facet on each stone; see vol. 1, page 176. The diamond bort is usually washed for its separation into two or three sizes, exactly after the manner of washing emery, except that the process is carried on upon a very much smaller scale, and the finest olive oil is used instead of water, the diamond powder is generally laid by under a stratum of oil to prevent waste; oil is employed because of its viscidity, it does not allow the diamond to subside so quickly as water, and it is moreover the fluid always employed in the using and preservation of the diamond by these artizans.
4. - The Application of Diamond Powder to the splitting or sawing of minerals will be described in the chapter XXXIV (Lapidary Work. Section I. - Slitting, Cutting, And Polishing Flat And Rounded Works). on Lapidary work. The coarser diamond powder used for grinding or cutting is generally burnished into the surface of the iron lap or skive of the diamond worker, and frequently also into the iron, copper, or other laps used by different artizans: in cutting sapphires the lapidary works the diamond powder into the copper lap, with a smooth piece of agate applied with gentle pressure. The finer diamond powder used for polishing, is simply applied on the surface of the tools, with the finger, or a small flattened wire used as a spatula. The gem engraver puts the diamond in minute hollowed disks of tin, two of which in fact are soldered to a strip of tin, and worn on the forefinger of the left hand as a ring: the one disk, of half an inch diameter, contains the mixed diamond paste, the other disk, one or two drops of the oil of brick, with which the tool is frequently lubricated.
5. - Fictitious Diamonds. - The white sapphire is sometimes used in jewellery as a substitute for the diamond, and the zircon is said also to be so employed when deprived of its colour by heat: the so-called Bristol diamonds are crystals of quartz cut and polished, but those imitations which are considered to come the nearest to real diamonds, in point of lustre or colour, though not in hardness, are met with amongst the pastes of the first quality, which are made artificially, and polished on pewter wheels with rottenstone, and not on copper wheels, like most of the hard gems.