Most cylindrical forms turned in soft steel, when subsequently hardened, more or less lose their axial truth in the process; they are in some cases "hacked" with the hammer, upon portions intentionally left soft, to bring those hardened back to truth, or the latter are ground true with emery, processes described in former volumes. The diamond affords a convenient and somewhat less tedious means of rendering these portions true.

The diamond is used to a limited extent as a hand tool, for turning such works as small hardened steel pivots, but more frequently in the slide rest. The hand tools are made of little fragments, split off from the stone in the first stage of its preparation as a gem, an angle of the fracture containing a portion of the external rind of the stone being preferred for the cutting edge; these pieces called "diamond bort," are inserted in the ends of brass or steel wires after the manner shown figs. 64 and 65 Vol. I., either in their split ends, or more generally in a hole pierced in the direction of the length of the wire. The stone being held by its largest part by pinching in the metal around it, by brazing, or sometimes by melted shell-lac. The tool cuts by the extreme point, and is constantly supplied with water, that neither it nor the work may heat. All the diamond tools produce the best results upon the hardest steel, less so upon portions less hard, and are inefficient upon soft steel, upon which they drag, compressing or burnishing a bright surface marked with occasional lines, rather than cutting. They require a very slow traverse and very slight depth of cut, to avoid risk of breaking the delicate point, and according to its diameter, a comparatively slow revolution of the work, less than half the usual speed for turning steel with the ordinary hand tools. Skilfully used, they may be made to turn absolutely hard steel to a true and perfectly polished surface or cylinder, removing the material in the form of minute turnings; caution however is requisite in the advance of the tool in turning the arris formed by these two; the intense hardness of both tool and work rendering this angle rather liable to partial fractures. For works of increased dimensions, a similar but larger fragment of the stone is brazed into a stout steel wire which is inserted in the end of a rectangular iron stem for the slide rest, and this enables the tool to be carried quite into an angle in the work; for plain cylinders, the stone is fixed directly in a hole made in the end of the iron stem.

Larger and stronger forms, different varieties of the diamond, which are remarkably permanent, are also employed. The Brazilian "ballas or ball bort" occurs in small quasi-spherical nodules, partially translucent, and also differs from the ordinary diamond in being an aggregation of crystals, possessing a radial instead of the octohedral fracture. Pieces about the size of a pea are placed in the end of an iron stem, brazed in, or the iron punched down around them, care being required in setting by the latter method to avoid cracking the diamond, very little of which is left exposed. Ballas cuts or rather abrades the steel by its rough exterior surface, formed by the ends of the collected crystals; its action is hardly so rapid as that of the previous tools, neither does it leave quite so good a surface, and as with all, it requires a constant supply of water.

"Carbonate," the hardest of the Brazilian diamonds, a noncrystalline substance, opaque, and ranging in colour from dark brown to nearly black, has an amorphous fracture ; some specimens contain numerous small irregular cavities, and under the microscope its appearance fairly resembles a piece of coke. Small pieces ranging in size from one to three carats, according to the magnitude of the work, are employed for turning steel, mounted as before in the end of an iron stem, leaving an angle of the fracture slightly projecting. The carbonate although the strongest of the tools, is still most efficient in the ordinary lathe when used like the others, with only a slight depth of cut, usually not exceeding one to two thousandths of an inch, the continued repetition of which gradually reduces the work to truth, leaving it smooth and highly polished. With increased depth of cut, the hardness of the work, its want of truth in revolution and the intense hardness of the tool, combine to exert so great leverage as to bring the latent elasticity of all parts of the apparatus into play; the tool continues to cut, but its effects are irregular and unsatisfactory. In more massive lathes the depth of cut may be proportionately increased, provided the work be also of sufficient stability, its effects upon the latter, as in all other turning, unerringly pointing out the extent to which the depth of cut may be safely carried. The material is ordinarily removed as minute particles, reduced copies of those produced by the graver; but in the course of experiments upon an absolutely hard steel cylinder, 5/8 ths of an inch diameter, made with a carbonate tool kindly placed at the author's disposal by Mr. J. Ker Gulland, C.E., the angle of the diamond was occasionally found to remove delicate spiral turnings about four inches in length, and but the two thousandth part of an inch in thickness. Which minute, but otherwise precise copies of the long spirals produced in ordinary iron turning in the slide lathe, may be considered as rather remarkable in turning hardened steel.

The carbonate among other purposes, is used in boring rock for tunnelling and well sinking, the drill driven by power, being virtually an enlarged example of that described, fig. 71. Vol. I.; the stones taking the place of the diamond powder. In the crown drill patented by the above named gentleman, the stones are placed in the ends of short, nearly cylindrical iron studs, which are fixed in holes bored in the annular face of a strong iron ring, arranged at intervals around both edges and over the surface; the studs being removable, that the same series of diamonds may be employed in rings of differing diameters as required. The largest drill yet made, and also constructed by Mr. Gulland, the Engineer of the Diamond Drill Company, is 23 inches in diameter, carrying fifty six diamonds, and is of the value of £500. This was used in boring to arrive at the green sand, in the deep well for the New River Company at Turnford, 1879; and the same diamonds were afterwards transferred to another ring 20½ in. diameter. The latter, working at a depth of 980 feet, and driven from the surface at about fifty revolutions per minute, was found to bore through the gault at the rate of from 12 to 18 inches in the hour; the water used in the process, at the same time so far washing away the crumbled margins of the hole, as to permit the concurrent sinking of the iron tubes 23 inches diameter, to form the lining of the well. Although comparatively soft, the gault and all other argillaceous formations are found to clog and impede the action of the tool, most of the harder strata being bored more rapidly.

Similar ballas and carbonate tools to those used for steel are largely employed in turning emery wheels and grindstones; water is always required, and with the latter entirely prevents the clouds of dust inevitable in turning grindstones by the usual method. The same tools have lately been successfully employed in turning granite, which material was otherwise incapable of being turned in the lathe. Vol. I. 171. The economy effected over the previous and tedious methods is very considerable, for the author is informed that a carbonate tool has been in constant use for this purpose for more than nine months; this stone has been occasionally reset for security, or to expose a fresh angle, but otherwise it remains quite serviceable. Carbonate, it may be mentioned, was discovered in the diamond gravels of Brazil in 1842, and its extensive use is recent; the valuable properties it possesses are perhaps equally testified by the fact, that so late as thirty years back its market price, now about twelve shillings for the inferior, to twenty five shillings the carat, for the finest quality, was but seven pence; unfortunately carbonate is not extensively met with.