on a steel plate revolving with great velocity-an operation perfectly analogous to that of glass cutting, or the ordinary well-known lapidary's wheel. The cutting plates are usually about ten or twelve inches in diameter; they are placed horizontally with their spindles vertical, and are made to revolve about thirty or forty times in a second; the part acting on the diamond travelling over the facet at the rate of about a mile in a minute. Diamond powder, of extreme fineness, mixed with the best olive oil, is placed with a feather upon the upper table of the wheel, and the apparatus is then ready for action on the diamond. The stone is embedded in a mass of soft metal, an amalgam of lead and tin, easily fusible, and yet hard enough to retain the stone firmly in its position; this is fixed in a moveable handle, which is again attached to a small frame. The workman, having first heated the metal to a soft state, beds the diamond in it in the required position, and fixes it there by plunging into water; the frame is then placed to project over the wheel, and the diamond, being downwards, comes in contact with its upper surface, on which the diamond powder is placed; weights are then applied, and the result of the friction, at the immense velocity, is to cut a facet upon the stone in a very short space of time. When one of these is finished, the workman softens the metal, extracts the stone, and replaces it in the proper position for making another facet; and here comes into play a very remarkable feature of the operation, namely, the accuracy of judgment which skill and experience give in arranging the faces of the stone. It is obvious that, in any many-sided solid body whose shape is to have any pretensions to regularity or symmetry, the different faces must not only all stand in certain definite angular positions in regard to each other, but must all bear a certain size in relation to the magnitude and form of the whole. Further, any one acquainted with geometry will know, that for a solid figure of fifty or sixty sides, the determination of these angles and surfaces, by any theoretical rule, would be a matter of great difficulty; while the attempt to make such a figure. practically, by any one unskilled in the operation, would only lead to continual trial-and error-attempts, which, even if the thing were ever properly done at all, would waste a large portion in the operation, and consequently much diminish the ultimate available size. Any one who will try, for example, to cut a turnip or a potato, by his eye and hand only, into a regular octohedron, or solid figure of eight equal and similar sides, will at once appreciate the difficulty. Yet the diamond-cutter has to do a much more difficult problem, namely, to give about sixty symmetrical and regular faces to stones sometimes only about an eighth of an inch diameter; without any mechanical aids whatever to his judgment; and yet producing, without a particle of unnecessary waste, the very largest stone geometrically possible out of the rough body. This of course can only be the result of great skill and long experience. Having made one facet, he judges by his eye the exact angle at which the stone must be placed to cut the new one, and the exact depth to which the grinding for the latter must be carried; and so accurately is this done, that it is very seldom a good workman ever has to revert to a facet for correction, after he has once passed it over. The stone is so fixed in the metal as to leave other facets visible for constant comparison with the one under progress; and the handle is capable, by a sort of universal joint, of adjustment to any nicety for the position of the stone in touching the wheel. There is no further division of labour than between the rough cutter and the finisher-the latter taking the stone from the former in its roughed-out state, and returning it to the proprietor in the shape of the perfect finished brilliant ready for sale. The last touches to the facets consist of polishing, or giving to them the peculiar diamond lustre ; but this is in no wise different from the grinding, except in being done with more care. The man can at any time adjust the weight or force with which the stone is pressed upon the wheel, or he can remove it entirely, and substitute the gentle pressure of his hand; and he can also modify the velocity of the grinding action; for, although the wheel itself is kept at a constant number of revolutions per minute, he can place the stone nearer to, or further from the axis, as he likes, which will of course give a less or greater effective velocity, according to the radius of the acting circle. The diamond powder, of which a large quantity is used, is obtained partly from the first process, of rough-cutting the stones; partly from diamonds of a quality not good enough to cut for sale, which are broken up for the purpose; and partly from the newly discovered substance, "carbonado," which is hard enough for this use, although of a somewhat coarse quality. The powder is carefully sifted, cleaned from dirt and extraneous matters, and, when about to be used, is mixed with the finest vegetable oil. The workmen are all Jews, and are regularly educated to the trade. They are paid by piece-work. Formerly, they did their work at their own houses, their wheels being turned by manual power; but it is now found more advantageous for the large proprietors to provide workshops of their own furnished with steam power, for the use of which the men pay out of their earnings. Some of the more skilful and industrious men realise considerable incomes. There is, of course, always temptation to dishonesty, from the great value which is compressed into so small a space; but all possible precautions are taken, and the character of the men is made of so high weight in all the transactions with them, that losses very seldom occur. The form into which a diamond is cut has great influence on its beauty and fire. The two most common are what are called the "brilliant," and the "rose" or "rosette." The latter, so named from its similarity to an unopened rosebud, was one of the earliest forms in use, and is applied generally to the cheaper kinds of stones. a pointed apex. The flat base is imbedded in the setting; and, therefore, in the rose diamond, the whole of the stone appears projecting above. The brilliant is the more valuable form; it may be considered as formed of two pyramids, connected together at their bases, with the apex of each truncated or cut off, and the sides worked into facets, as in the case of the rose. The stone is held in the setting at the broadest part, or junction of the pyramids; pyramids; one pyramid projects upwards in sight, the other is hidden below, so that only half the stone, or somewhat less, appears; but the hidden part is most powerfully effective in adding to the brilliancy. The apex of the upper pyramid is cut off to a considerable extent, and the large facet thus formed is called the table: the corresponding facet below, formed by the truncation of the lower or hidden pyramid, is much smaller, and is called the collet. The rim where the setting takes hold, or, as we have described it, the junction of the bases of the pyramids, is called the girdle. There are thirtytwo facets cut round the upper slanting surface of the stone, i.e., between the girdle and the table, and twenty-four on the lower part, between the girdle and the collet. All these facets have names by which they are known to the cutters; and all the dimensions of the stone should, in order to produce the best effect, bear certain definite proportions to each other. The most favourable form of brilliant for exhibiting the lustre of the stone is considered to be a square, having the corners slightly rounded off; but, of course, many stones will not admit of being cut to this form without loss, and, therefore round, oval, pear shapes, &c., are perhaps more common. The stones lose about fifty per cent. in cutting, more or less, so that, to make a brilliant of one carat, a rough stone of two carats is required. The chemical nature of the diamond is well known. It consists of pure It is a carbon; identically the same thing as the soot from a kitchen chimney, but in different form. Sir Isaac Newton sus sort of pyramid, with a flat base, and inclined facets, terminating upwards in pected, by its optical properties, that it was a combustible body; and its character has been subsequently proved beyond a doubt. If sufficient heat be applied, diamonds will completely consume, combining with oxygen to form carbonic acid, precisely like charcoal or coke in an ordinary furnace. There have been many speculations as to the mode by which nature has effected this wonderful metamorphosis, and many have been the attempts made to imitate her; but hitherto she has kept her secret well, and baffled all her admiring followers. Sir David Brewster has suspected, by optical peculiarities exhibited in some examples, that diamonds may not be of mineral origin, but may have resulted from the hardening of a kind of gum, something like amber. A curious substance has lately been found in the Brazilian mines, called "Carbonado," or amorphous diamond-a kind of intermediate grade between diamond and charcoal, combining the hardness of the former with the black unformed character of the latter. Close inspection shows curious traces of a passage between the two states; and it is thought further examination of this substance may lead to some better insight than we at present possess, as to the chemical nature of the change. The diamond is totally insensible to the action of any chemical reagents. Its specific gravity is about 3.5. The most characteristic quality of the diamond is its extreme hardness; it is the hardest substance known. This quality was the earliest that attracted attention, the name being derived from the Greek 'Adápas, i. e. incapable of being crushed or subdued For the comparison of hardness in different degrees, mineralogists have adopted a scale represented by the following substances. 1, tale; 2, gypsum; 3, calcareous spar ; 4, fluor spar; 5, phosphate of lime; 6, felspar; 7, quartz; 8, topaz; 9, sapphire and ruby; 10, diamond. Any one of these substances will scratch all below it in the scale, and may be scratched by all above it. The dia mond, therefore, as far as destructibility by abrasion is concerned, defies all nature. This quality renders it of considerable value for other purposes than ornament—as for cutting glass, and for working other stones, for the pivots of watch-work, &c. But, although the diamond is so hard, it is very easily broken, and, indeed, by a particular knack, it may even be cut with a common pen-knife. This apparent anomaly is due to what is called its cleavage, a result of the crystalline structure. Many well-known substances, as slate for example, split or cleave with peculiar facility in certain definite directions, while they offer considerable resistance to fracture in all others. The diamond has this property, cleaving easily in no less than four directions, parallel to the surfaces of the original octohedric crystal; and, therefore, when moderate force is applied in either of these ways, the stone splits into pieces. Pliny, mentioning the great hardness of the diamond, states that if laid upon an anvil, and struck with a hammer, the steel would sooner give way than the stone. This assertion is a matter of popular belief in the present day, but we would not recommend any possessor of a good diamond to try the experiment. The chances of some of the forces acting in the cleavage directions are so great, that the stone would in all probability fly to pieces under the first blow. The truth is, that Pliny referred not to the diamond, but to the sapphire, which, though less hard than the diamond, cleaves only in one direction, and might, therefore, withstand the test named. The cleaving property of the diamond is made useful in two ways in the manufacture first, by splitting the stones when they contain flaws, and secondly, in the preparation of diamond powder. When a rough diamond is seen to contain a defect of sufficient extent to depreciate its value as a single gem, it is split in two, precisely at the flaw, so as to make two sound stones. This is a very simple operation in appearance, done in a few seconds; but it requires an amazing amount of skill to do it properly. The workman, by a sort of intuitive knowledge, gained by long experience, knows, on a careful inspection of the stone, the exact direction which a cleavage plane passing through the flaw will take. Tracing this plane therefore to the exterior, he makes on the edge of the stone, precisely in that spot, a slight nick with another diamond. He then places a small knife in that nick, gives it a light tap with a hammer, and the stone at once cleaves in two, directly through the flaw. This operation, in daily practice in the Amsterdam works, is one of the most elegant and instructive processes in the whole range of mineralogy. It is reported that Dr. Wollaston, celebrated as almost the originator of the science of crystallography, once made a handsome sum by purchasing a large flawed diamond from Rundall and Bridge at a low price, and subsequently splitting it into smaller sound and valuable stones; the principle of the operation not being then generally known. Another use of the cleavage principle is in the preparation of diamond powder. Small diamonds of inferior quality, are put into a steel mortar, and pounded and rubbed with a steel pestle, when they break up through their various cleavage planes into still smaller pieces, and at last rub themselves into the finest dust, fit for use on the wheel. The cause of the wonderful brilliancy of the diamond is not popularly known. It has no inherent luminous power; it is simply transparent, like common glass, and yet, if the latter were cut into the form of a brilliant, it could no more be mistaken for a real one than for a sapphire or an emerald. The secret, therefore, of the brilliancy of the diamond must lie in something other than its clearness or its transparency. It is owing to its great refractive power. When rays of white light pass through transparent substances they are fracted, or bent out of their former course, and under certain circumstances are separated into their constituent elements, and dispersed in the form of the re well-known prismatic colours. The cut drops of glass chandeliers show a familiar example of these properties. Now, the degree in which this effect is produced by any substance depends on the refractive power it possesses, and it so happens that the diamond has this power in an extraordinarily high degree, its index of refraction being 2:47, while that of glass, or rock crystal, is only about 16, and of water 13. The effect of this great refractive capability, particularly when aided by judicious cutting, is, instead of allowing the light to pass through, to throw it about, backwards and forwards in the body of the stone, and ultimately to dart it out again in all sorts of directions, and in the most brilliant array of mingled colours; and this is this marvellous effect that meets the eye. Sir David Brewster has shown1 that the play of colours is enhanced by the small dispersive power of the diamond, in comparison with its refractive properties. It is often supposed that diamonds are essentially colourless, but this is a mistake; they exist of many colours, yellow, orange, pink, blue, green, brown, and black. Three-fourths of the stones found are tinged with some colour or other, mostly pale yellow, or yellow brown. The perfectly pure and colourless ones are selected as the most valuable for the general market; but it sometimes happens that fine stones of a decided colour are more prized than white, from their peculiar rarity and beauty.2 A blue diamond of about fiftysix carats, belonging to Mr. Hope, is a celebrated stone, combining the beautiful colour of the sapphire with the fire and brilliancy of the diamond. The quality of diamonds depends upon their colour, purity, transparency, and freedom from flaws. Stones perfectly colourless, pure, clear, and free from all defects, are said to be of "the first water;" if they have slight imperfections, they are "of the second water; 1 North British Review, Nov. 1852. 2 A fine collection of coloured diamonds, belonging to Mr. Tennant, are now exhibiting at the Kensington Museum. and, if tinged with colour, or otherwise very defective, of "the third water." The value is estimated according to the weight, which is expressed in carats; one carat being about 205 French milligrammes, or 3 grains troy. For small stones, not exceeding one carat in weight, the value may be assumed approximately to be proportional to the weight; but, as the stones increase in size, this rule does not apply the larger ones being more rare, and therefore having a value greater than is due to their mere size. To provide for this, it is generally assumed that, above one carat, the value shall increase as the square of the weight-i.e., that a stone double the weight of another shall have four times the value; treble the weight, nine times the value; ten times the weight, one hundred times the value, and so on. The money value of diamonds is a difficult subject to touch upon, as a distinction must always be drawn between the retail price asked by jewellers from the public, and the real market price of the diamonds as sold by the dealers. Moreover, the value will always vary according to the state of the market, as well as according to the quality and cut of the stones. As a rough approximation, brilliants of firstrate quality, and perfect in every respect, may be estimated at about 127. per carat; reducible to half this, or even less, for stones of inferior water. According, therefore, to the rule of the weight above laid down, a diamond of half a carat might be estimated as worth 64.; but one of two carats would be worth 2 x 2 x 12 = 481; one of five carats 5 x 5 x 12 3007; and so on.1 1 Referring to the square or best form of brilliants, the solid content of a cut stone, of proper proportions, is about of that of the circumscribing parallelopipedon; and, taking the Sp. gr. at 35, we shall obtain the following rule. Let d side of the square, or breadth across the girdle, and t the thickness of the stone, from table to collet; both in tenths of an inch;-then This rule will, however, hold only up to the limit of stones in ordinary sale. Such as are very large and of exceptional production cannot be valued by any rule; they are worth just what the state of the demand among crowned heads and millionaires will enable their holders to get for them. The general value of diamonds has been rising of late years; for, though the production is not scanty, the demand, owing to general prosperity, and the extension of ornament to wider classes in society, is largely on the increase. Imitations of diamonds are generally of one of the following three kinds : 1. White Topaz.-This is nearly as hard as diamond, and about the same specific gravity, and may therefore be mistaken for it when tried by these tests. A London jeweller died lately in the belief that a fine stone he had come into the possession of was a valuable diamond, and left large legacies to be paid out of the proceeds of its sale; but it proved, on examination, to be only a white topaz, and of very little value. The difference may be recognised by the optical qualities, which differ much in the two stones. 2. Rock Crystal (Brighton diamonds, Irish diamonds, &c.).-This substance, though hard enough to scratch glass, is much softer than diamond, and is easily scratched by it. It is also much inferior in brilliancy and in specific gravity. 3. Paste. This, which is a glass prepared with metallic oxides, can be made equal to diamond in refractive power, and therefore can be given a great brilliancy; but it is very soft, softer even than common glass, and it does not retain its lustre. There is also a method of deception sometimes practised by what is called half-brilliants; i. e. stones in the form |