A Treatise on Diamonds, and Precious Stones/Chapter 1/Section 8

by John Mawe

SECT. VIII.

Art of Cutting, Splitting, Sawing, and Polishing Diamonds^[1].

The object of cutting and polishing the Diamond is twofold. First, to divide the natural surface of the stone in a symmetrical manner, by means of a number of highly polished polygonal planes (facets), and thus to bring out to the best advantage the wonderful refulgence of this beautiful gem; and secondly, by cutting away such flaws as may happen to be near the surface, to remove those blemishes that materially detract from its beauty, and consequently from its value.

The removal of flaws is a matter of great importance, for, owing to the form in which the diamond is cut, and its high degree of refrangibility, the smallest fault is infinitely multiplied, especially if near the collet^[2], and becomes obtrusively visible in every facet.— For this reason also, it is by no means an easy matter, at all times, to ascertain whether a flaw is, or is not superficial; but a person of experience will often purchase to great advantage stones appearing to be full of flaws, which in fact exist only on the surface.

The principal object of the artist, when a rough diamond is put into his hands, is to examine carefully in what direction the stone ought to be cut, and where to form the girdle or spread of the brilliant. So much stress is laid by modern fashion on the superficial extent of a brilliant, that the old rules for proportioning its dimensions are now nearly obsolete: the diamond-cutters have almost discarded the use of measures, and, in forming the facets, trust wholly to the eye. If, however, the brilliant were formed according to the rules, it would be in the best proportion, and exhibit the greatest possible refulgence. At present it is the practice in cutting a diamond almost exclusively to consider which form the rough stone is best calculated to produce, without any regard to scientific accuracy.

As the octahedron is the most common form of rough diamonds, I will illustrate what Ihave to observe on the art of culling, by supposing such a diamond, which is technically called a six-cornered stone, to be given to a workman to make into a brilliant. The tabie and collet are cut upon opposite solid angles, in a direction perpendicular to the axis of the crystal, and the facets obliquely, upon the lateral edges, and other four angles. It will be evident that the labor required to form the table and collet, must be considerably greater than is necessary for the facets, as the artist has to work directly across the lamine: hence these are called the hard points. In working upon the other angles, a facet is produced with much less difficulty, it being cut in an oblique direction to the lamine; in fact, if the same force were exerted upon them as is applied to the others, perhaps a flaw would be produced. It is probably on account of this nice adaptation of force required for the various parts of the operation, that the fatiguing process of manual labor has not been superseded by machinery.

The operation of cutting is commenced by imbedding the diamond in strong cement, fixed at the end of a stout spindle-shaped stick about a foot long, with that portion only projecting, the removal of which is to form the first facet. Another diamond^[3] is employed, for this purpose, fixed as the former, with one of the solid angles projecting. To collect the powder and shivers that are detached during the process, the cutting is performed over a strong box, five or six inches long, furnished with a false bottom, perforated with excessively minute holes, in order to sift, as it were, the dust from the shivers; and also with two upright iron pegs fixed on the sides, for the workman to support and steady the sticks and his fingers against, while with a short repeated stroke, somewhat between rubbing and cutting, he is laboriously wearing away the diamond in that part where the facet is to be placed. This being done, the cement is softened by heat, and the position of the diamond is changed, in order to bring a fresh part under the operation of cutting. When all the facets have been placed upon the surface of the diamond, the cutting is completed. The stone, if examined, now presents rough edges and dull surfaces, resembling a piece of unpolished glass: these imperfections are removed by polishing, which gives the greatest sharpness to the angles.

The shape of many Diamonds is so irreguJar, that it is necessary to remove pieces of considerable magnitude, in order to bring them to a form proper for cutting. Where the lines of these proposed sections coincide with the lamellar structure of the stone, the workman has recourse to the delicate and perhaps somewhat hazardous operation of splitting, by which a double advantage is obtained. In the first place there is a great saving of time, and in the second place the pieces are themselves sufficiently large to admit of being cut and polished. The method of splitting is made a great mystery of: thus much however may be mentioned, that when the direction in which the section is to be made has been determined on, it is marked by a line cut by a sharp^[4]; the stone is afterwards fixed by strong cement in the proper position in a stick, and then, by the application of a splitting knife, the section is effected by a smart blow.

It sometimes happens that the foul and flaws occur in a part of the diamond which cannot be split off, the laminz lying in a contrary direction; recourse must then be had to sawing, which is performed as follows—The diamond is cemented to a small block of wood, which is firmly fixed to the table, and a line is made with a sharp where the division is intended to take place, which is afterwards filled with diamond-powder and olive-oil: the sawing is then commenced, and if the stone be large, the labor of eight or ten months is sometimes required to complete the uperation. The saw is made of a fine wire of brass or iron, attached to the two ends of a piece of cane or whalebone, the teeth being formed by the particles of diamond-powder, which become imbedded in the wire, as soon as it is applied to the line. In large ill-shaped stones, sawing is practised to advantage, but this method. is not advisable, unless the part to be removed is valuable.

The Diamond being thus brought to the required form, the next object is to polish the facets. The polishing mill is an extremely simple machine, consisting of a circular horizontal plate of cast iron, fourteen or fifteen inches in diameter, (called a skive,)^[5] fixed on aspindle, and capable of being put into rapid motion by means of a larger wheel, five or six feet in diameter, and turned by an assistant. In order to -keep the Diamond perfectly steady while the polishing: of each facet is going on, the following contrivance is had recourse to. .A copper cup (called a dopp,) about three quarters of an inch in depth and the same in width, and furnished with a stem about four inches long, of stout copper wire, is filled with plumber's solder, which projects in a conical form beyond the rim of the cup: in the apex of this cone, the solder being softened by heat, the diamond is imbedded with the facets to be polished projecting. The stem of the cup is now put into very powerful pincers (tongs), not unlike snuffers, which screw up with a nut and a wrench or lever, and thus hold it perfectly tight. In this position the diamond is carefully placed on the skive, the pincers resting on their legs on the bench that supports the mill, and pressing at the same time against an upright iron peg; the broad part of the tongs is next loaded with weights, to increase the pressure of the diamond upon the skive. The skive being previously set in motion, at the rate of about 200 revolutions ina minute, the process of polishing is begun. The diamond is taken up and examined from time to time, and is adjusted so as give the facet its true form^[6]. The heat occasioned by the friction is at all times considerable, and when the pincers are heavily loaden, as in polishing large stones, it occasionally increases to such a degree as to soften the solder and displace the diamond; when this accident occurs, it often produces a flaw, and always tears up the surface of the skive. Three or four diamonds may be polished at the same time; and to give each its proper share of attention, is as much as one person can well manage^[7].

Diamonds are brilliant cut, rose cut, table cut, and lasques. The brilliant is deservedly in the highest estimation, as it is the form which shows to the greatest advantage the peculiar lustre of the gem. The brilliant ^[8] may be considered as formed of two truncated pyramids united together by one common base, the upper pyramid being much more deeply truncated than the lower. The plane formed by the truncature of the upper pyramid is called the table (a); that formed by the truncature of the lower is called the collet (b); the common base is called the girdle (c); the space between the table and the girdle is the bizel (d), and that between the girdle and the collet is the collet-side (e). The inclination of the facets to the girdle ought to be 45°, and the bizel should be inclined to the table at the supplement of the same angle; it is absolutely necessary that the collet should be exactly parallel to the table.—If these measurements are accurately observed, the superlative splendor of the diamond will be displayed to the greatest advantage. The rule tobe adopted in regulating the height of the brilliant is, (supposing the stone to be a regular octahedron), to divide it into eighteen parts. Five-eighteenths are cut away to form the table, and one-eighteenth for the collet, which will reduce the height one-third, and the diameter of the collet will be one-fifth of the table. If these distances are preserved, the collet will play in the centre of every facet; but if there be any variation, it will play higher or lower, and greatly diminish the intensity of lustre. The corners are truncated, to correspond with the table; the bizel is formed by eight lozenges and twenty-four triangles, and the collet-side by four large irregular pentagons, and four lozenges, radiating from the collet as a centre, and bordered by sixteen triangles at the girdle; thus making, in the whole, fifty-six facets^[9]. The triangles on the bizel, adjacent to the girdle, are called skill facets, and those which join the table, star facets; those upon the collet-side are called under skill facets, and the sides or pentagons, pavilion facets.

Many of the foreien-cut brilliants have the facets inclined at an angle of 50°, or even more, the principal study with foreign workmen being to retain as much weight as possible.—These stones are sometimes reduced to the above standard by English artists, it being found that the additional expense in recutting the facets and the consequent loss of weight is fully compensated by the increased brilliancy of the gem. The brilliant is set with the table side upwards, and the collet-side implanted in the cavity made to receive the diamond.

The regular rose diamond^[10], is the form given to those stones, the spread of which is too great in proportion to their depth to admit of being brilliant cut. It is formed by covering the rounded surface of the stone with equilateral triangles, placed base to base, making the figure of a rhomb.

The table diamond^[11] is the least beautiful except the lasques, and is made of those stones, which, witha considerable breadth, have only a very trifling depth. It is produced by a series of diminishing four-sided planes below the girdle; and the bizel is formed by one, two, or three of the same.

Lasques^[12] are formed from flat or veiny diamonds. They are quite unknown to European workmen, and are only made in India.

The polishing of the girdle should be particularly attended to in all these modes of cutting, as a rough edge, when seen through some of the facets, has often the appearance of a flaw, and diminishes the brilliancy of the stone: and unless the girdle is perfectly smooth, thin diamonds are liable to be damaged by the pressure required to secure them in their setting.

Diamonds from one carat to four or five carats, when skilfully cut into brilliants, lose in the process a little less than half their weight: hence the value of a cut stone is twice that of a rough one of equal weight, exclusive of the cost of workmanship.

The diamond cutters of England are confessedly the best in Europe, but the Dutch have engrossed almost all the trade, as we are unable to compete with their prices, labor being so much lower in Holland.

Diamonds are always equally in fashion but the mode of setting varies according to caprice, or the desire of novelty: hence the jeweller has perpetual opportunities of exercising and displaying the inventive elegance of his taste^[13].

1. The same workman is seldom a proficient in all these operations, but generally confines himself to one.
2. See page 77.
3. An inferior diamond, or a piece of bort, is generally used; but a skilful workman, if he has a lot to cut, will select those stones which suit his purpose, and so employ one against another, as to form two at the same time. The average work of a diamond-cutter is two carats per diem; but to polish the same requires twice that time. The usual price of cutting and polishing is 14s. per carat,
4. When a small diamond is broken into four parts, the edge of the quadrant is called a sharp.
5. The surface of the plate is roughened by the application of a grit-stone, in order that it may better retain the mixture of olive-oil and diamond-powder, with which the skive is charged, and which is renewed, as occasion requires.
6. When the facet is cut exactly in the direction of the lamin, it will not receive a polish; in this case the stone must be inclined.
7. Formerly, both in India and Europe, before the art of cutting was discovered, diamonds were worn down upon the skive. 'When the mines of Golconda were in their zenith, 400 skives were constantly employed upon the spot.
8. See plate I. fig. 4.
9. See plate I. fig. 5 and 6.
10. See plate I. fig. 7.
11. See plate I, fig. 10.
12. See plate I, fig. 11, 12.
13. It is customary with jewellers when they receive an order to make a particular design, to fix the diamonds in black or white wax in the form required.—If they are breathed upon until the glare or lustre is destroyed, any imperfections may be easily discovered.