These may be split with plane surfaces through their natural cleavages, and which method is continually employed; otherwise, they can be only slit with the diamond powder. The first and similar stones may be smoothed with emery, but emery being in hardness only equal to 9, produces but little effect upon topaz, upon sapphire and ruby it is almost inert, and on diamond quite so; the sapphire and ruby and also diamonds arc therefore always polished with diamond powder.
On account of the peculiar interest attached to the mechanical applications of the hard gems, it is proposed to depart a little from the subject and order of these pages, to advert to some few of their uses, which may not be generally understood. The sapphire, the ruby and also the diamond, are commonly used for the construction of certain parts of the best time-pieces and watches, such as the pivot-holes, pallets, and other parts of the escapements.
Fig. 58 represents upon a true, although very enlarged scale, the jewelled pivot-hole for the verge, or the axis of the balance of a marine chronometer, a is the hardened steel pivot, which is turned with a fine cylindrical neck, and made convex at the end; the jewelling consists mostly of two stones, the one, commonly sapphire or ruby, is turned to the form of the black figure b, that is, convex above and concave beneath, of two different sweeps, to thin it away at the part where it is to be pierced with the hole, and which is made a little smaller in the middle to lessen the surface bearing.
The other, which is called the "top-stone" or "end-stone," is generally a ruby, in the form of a plano-convex lens, or else it is a diamond cut into facets; the flat side of this touches the end of the pivot.
Each stone is burnished into a brass or steel ring, like some of the lenses of telescopes, and the two stones, (separated a slight distance for the retention of oil by capillary attraction,) are inlaid in a counter-sunk recess, c, d, in the side plate, or other part of the watch, and retained therein by two side screws as represented, although unimportant variations are made by different artists in the shapes and proportions of the parts.
The delicacy of these jewelled holes will be imagined, when it is added that in the axis above referred to, the side plate e, e, is only one-tenth of an inch thick; the rings from c, to d, one-sixth, and the pivot the one two-hundredth part of an inch diameter; in pocket-watches, and more particularly the flat Geneva watches, these measures, especially the first two, are amazingly reduced, although the same number of parts are nevertheless employed in every hole that is jewelled.
The wire for making the pendulum springs for chronometers, is sometimes drawn through a pair of flat rubies with rounded edges, as represented in fig. 59; the stones are cemented into the ends of metal slides having screw adjustments, not represented. Sometimes two pairs of rubies are placed one before the other, to constitute a rectangular hole of variable dimensions, for equalizing the wire both in width and thickness.
Rubies and other gems, are drilled with holes conical from both sides, as in fig. 60, for drawing the slender silver gilt, and silver wires used in the manufacture of gold and silver lace; the wires are afterwards flattened, wound spirally upon silk, and then woven into the lace. Ruby holes are also employed for rounding the leads of ever-pointed pencils; but for this use they are chamfered from the one side only, and the lead is pushed through from the small side, the ruby is then used as a cutting tool; whereas the hole in the draw-plate is slightly rounded upon the ridge, and acts more as a burnisher or compresser; the action of the wire, which is pulled through in the direction of the arrow, tends to draw the stone more firmly into its sent. The finest holes of all, are made by barely allowing the point of the drill to penetrate into the apex of the conical hole, previously formed on the opposite side of the ruby.
All these applications are adopted on account of the very great hardness of the stone, but they could scarcely exist were there not one substance still harder than the ruby to serve for the tools by which these several forms are wrought, and the brief consideration of which will now be proceeded with.
The diamond is the hardest substance in nature, and in common with some other crystalline bodies, it is harder at the natural angles and edges, and also at the natural coat or skin of the stone than within, or in its general substance. Its peculiar hardness is probably altogether due to its highly crystalline form, as by analysis the diamond, charcoal, and plumbago, are found to be nearly identical: the first is absolutely pure carbon, the others are nearly so.*
The principal use of the diamond is for jewellery, its preparation for which will be touched upon in the slightest possible manner; but from its peculiar hardness the diamond fulfils some more really important although less brilliant services as tools, without which several curious and highly valuable processes must be altogether abandoned, and others accomplished in an inferior although more costly manner by other means.
The diamond is prepared for the purposes of jewellery by three distinct processes, namely splitting, cutting and polishing, which will be adverted to in a very few lines. In order to split off the portions not required, the stone is fixed in a ball of cement, about as large as a walnut, the line of division is sawn a little way with a pointed diamond fixed in another ball of cement, and the stone is afterwards split with the blunted edge of a razor struck with a hammer; the small fragments removed, when they are too small for jewellery, are called Diamond bort.