The description of the properties of diamond would not be complete without a reference to the other valuable, if utilitarian, purposes to which it is put. Without its aid much of modern engineering work and mining operations would be impossible except at the cost of almost prohibitive expenditure of time and money.
Boring through solid rock has been greatly facilitated by the use of the diamond drill. For this purpose carbonado or black diamond is more serviceable than single crystals, and the price of the former has consequently advanced from a nominal figure up to £3 to £12 a carat. The actual working part of the drill consists of a cast-steel ring. The crown of it has a number of small depressions at regular intervals into which the carbonados are embedded. On revolution of the drill an annular ring is cut, leaving a solid core which can be drawn to the surface. For cooling the drill and for washing away the detritus water is pumped through to the working face. The duration of the carbonados depends on the nature of the rock and the skill of the operator. The most troublesome rock is a sandstone or one with sharp differences in hardness, because the carbonados are liable to be torn out of their setting. An experienced operator can tell by the feel of the drill the nature of the rock at the working face, and by varying the pressure can mitigate the risk of damage to the drill.
The tenacity of diamond renders it most suitable for wire-drawing. The tungsten filaments used in many of the latest forms of incandescent electric lamps are prepared in this manner.
Diamond powder is used for cutting and turning the hardened steel employed in modern armaments and for other more peaceful purposes.
Although nearly all the gem-stones scratch glass, diamond alone can be satisfactorily employed to cut it along a definite edge. Any flake at random will not be suitable, because it will tear the glass and form a jagged edge. The best results are given by the junction of two edges which do not meet in too obtuse an angle; two edges of the rhombic dodecahedron meet the requirements admirably. The stones used by the glaziers are minute in size, being not much larger than a pin’s head, and thirty of them on an average go to the carat. They are set in copper or brass. Some little skill is needed to obtain the best results.
The value of a diamond has always been determined largely by the size of the stone, the old rule being that the rate per carat should be multiplied by the square of the weight in carats; thus, if the rate be £10, the cost of a two-carat stone is four times this sum, or £40, of a three-carat stone £90, and so on. For a century, from 1750 to 1850, the rate remained almost constant at £4 for rough, £6 for rose-cut, and £8 for brilliant-cut diamonds. Since the latter date, owing to the increase in the supply of gold, the growth of the spending power of the world, and the gradual falling off in the productiveness of the Brazilian fields, the rate steadily increased about 10 per cent. each year, until in 1865 the rate for brilliants was £18. The rise was checked by the discovery of the South African mines; moreover. since comparatively large stones are plentiful in these mines, the rule of the increase in the price of a stone by the square of its weight no longer holds. The rate for the most perfect stones still remains high, because such are not so common in the South African mines. The classification[7] adopted by the syndicate of London diamond merchants who place upon the market the output of the De Beers group of mines is as follows:—(a) Blue-white, (b) white, (c) silvery Cape, (d) fine Cape, (e) Cape, (f) fine bywater, (g) bywater, (h) fine light brown, (i) light brown, (j) brown, (k) dark brown. Bywaters or byes are stones tinged with yellow.
The rate per carat for cut stones in the blue-white and the bywater groups is:—
| Blue-White. | Bywater. | |
|---|---|---|
| 5-carat stone | £40–60 | £20–25 |
| 1 „ | 30–40 | 10–15 |
| ½ „ | 20–25 | 8–12 |
| ¼ „ | 15–18 | 6–10 |
| Mêlée | 12–15 | 5–8 |
Mêlée are stones smaller than a quarter of a carat. It will be noticed that the prices depart largely from the old rule; thus taking the rate for a carat blue-white stone, the price of a five-carat stone should be from £150–200 a carat, and for a quarter-carat stone only £7, 10s. to £10 a carat. There happens to be at the time of writing very little demand for five-carat stones. Of course, the prices given are subject to constant fluctuation depending upon the supply and demand, and the whims of fashion.