Gem-Stones and Their Distinctive Characters

by George Frederick Herbert Smith

Fig. 52.—Step- or Trap-Cut (side view).

A popular style of cutting which is much in vogue for coloured stones is the step- or trap-cut, consisting of a table and a series of facets with parallel horizontal edges (Figs. 51–52) above and below the girdle; in recent jewellery, however, the top of the stone is often brilliant-cut. The contour may be oblong, square, lozenge, or heart-shaped, or have less regular forms. The table is sometimes slightly rounded. Since the object of this style is primarily to display the intrinsic colour of the stone and not so much a brilliant play of light from the interior, no attempt is made to secure total-reflection at the lower facets. The stone therefore varies in depth according to its tint; if dark, it is cut shallow, lest light be wholly absorbed within, and the stone appear practically opaque, but if light, it is cut deep, in order to secure fullness of tint. Much precision in shape and disposition of the facets is not demanded, and the stones are usually cut in such a way that, provided the desired effect is obtained, the weight is kept as great as possible; we may recall that stones are sold by weight. In considering what will be the optical effect of any particular shape, regard must be had to the effective colour of the transmitted light. For instance, although sapphire and ruby belong to the same species and have the same refractive indices, yet, since the former transmits mainly blue and the latter red light, they have for practical purposes appreciably different indices, and lapidaries find it therefore possible to cut the base of ruby thicker than that of sapphire, and thus keep the weight greater. It is instructive too what can be done with the most unpromising material by the exercise of a little ingenuity. Thus Ceylon sapphires are often so irregularly coloured that considerable skill is called for in cutting them. A stone may, for instance, be almost colourless except for a single spot of blue; yet, if the stone be cut steeply and the spot be brought to the base, the effect will be precisely the same as if the stone were uniformly coloured, because all the light emerging from the stone has passed through the spot at the base and therefore been tinted blue.

The mechanism employed in the fashioning of gem-stones is simple in character, and comprises merely metal plates or wheels for slitting, and discs or laps for grinding and polishing the stones, the former being set vertically and rotated about horizontal spindles, and the latter set horizontally and rotated about vertical spindles. Mechanical power is occasionally used for driving both kinds of apparatus, but generally, especially in slitting and in delicate work, hand-power is preferred. In the East native lapidaries make use of vertical wheels ([Plate XIII]) also for grinding and polishing stones, which explains why native-cut stones never have truly plane facets; it will be noticed from the picture that a long bow is used to drive the spindle.

Owing to the unique hardness of diamond it can be fashioned only by the aid of its own powder. The process differs therefore materially from the cutting of the remaining gem-stones, and will be described separately. Indeed, so different are the two classes of work that firms seldom habitually undertake both.

The discovery of the excellent cleavage of diamond enormously reduced the labour of cutting large stones. A stone containing a bad flaw may be split to convenient shape in as many minutes as the days or even weeks required to grind it down. The improvement in the appliances and the provision of ample mechanical power has further accelerated the process and reduced the cost. Two years were occupied in cutting the diamond known as the Pitt or Regent, whereas in only six months the colossal Cullinan was shaped into two large and over a hundred smaller stones with far less loss of material.

Although the brilliant form was derived from the regular octahedron, it by no means follows that, because diamond can be cleaved to the latter form, such is the initial step in fashioning the rough mass. The aim of the lapidary is to cut the largest possible stone from the given piece of rough, and the finished brilliant usually bears no relation whatever to the natural octahedron. The cleavage is utilized only to free the rough of an awkward and useless excrescence, or of flaws. Although the octahedron is one of the common forms in which diamond is found, it is rarely regular, and oftener than not one of the larger faces is made the table.

The old method, which is still in use, for roughly fashioning diamonds is that known as bruting, from the French word, brutage, for the process, or as shaping. Two stones of about the same size are selected, and are firmly attached by means of a hard cement to the ends of two holders, which are held one in each hand, and rubbed hard, one against the other, until surfaces of the requisite size are developed on each stone. During the process the stones are held over a small box, which catches the precious powder. A fine sieve at the bottom of the box allows the powder to fall through into a tray underneath, but holds back anything larger. By means of two vertical pins placed one on each side of the box the holders are retained more easily in the desired position, and the work is thrown mainly on the thumbs. This work continued day after day has a very disfiguring effect upon the hands despite the thick gloves that are worn to protect them; the skin of the thumbs grows hard and horny, and the first and second fingers become swollen and distorted. When the surfaces have thus been formed, the stone is handed to the polisher, who works them into the correct shape and afterwards polishes them, the stone passing backwards and forwards several times between the cutter and the polisher. The table, four templets, culet and four pavilions are first formed and polished, so that the table has a square shape. Next the quoins are developed and polished, and finally the small facets are polished on, not being shaped first. In modern practice the process of bruting has been modified in some cases by the introduction of machinery, and the facets are ground on, with considerable improvement in the regularity of their size and disposition, and reduction in the amount of polishing required. Moreover, to obviate the loss of material resulting from continued grinding, large stones are first sliced by means of rapidly-revolving copper wheels charged with diamond powder.

The laps used for polishing diamonds are made of a particular kind of soft iron, which is found to surpass any other metal in retaining the diamond powder. They are rotated at a high rate of speed, which is about 2000 to 2500 revolutions a minute, and the heat developed by the friction at this speed is too great for a cement to be used; a solder or fusible alloy, composed of one part tin to three parts lead, therefore takes its place. The solder is held in a hollow cup of brass which is from its shape called a ‘dop,’ an old Dutch word meaning shell. Its external diameter is ordinarily about 1½ in. (4 cm.), but larger dops are, of course, used for large stones. A stout copper stalk is attached to the bottom of the dop; it is visible in the view of the dop shown at e on [Plate VI], and two slabs of solder are seen lying in front of the dop. The dop containing the solder is placed in the midst of a non-luminous flame and heated until the solder softens, when it is removed by means of the small tongs, c, and placed upright on a stand such as that shown at a. The long tongs, d, are used for shaping the solder into a cone at the apex of which the diamond is placed. The solder is worked well over the stone so that only the part to undergo polishing is exposed. A diamond in position is shown at f. The top of the stand is saucer-shaped to catch the stone should it accidentally fall off the dop, and to prevent pieces of solder falling on the hand. While still hot, the dop with the diamond in position on the solder is plunged into cold water in order to cool it. The fact that the stone withstands this drastic treatment is eloquent testimony to its good thermal conductivity; other gem-stones would promptly split into fragments. It may be remarked that so high is the temperature at which diamond burns that it may be placed in the gas flame without any fear of untoward results. The dop is now ready for attachment to an arm such as that shown at b; the stalk of the dop is placed in a groove running across the split end of the arm, and is gripped tight by means of a screw worked by the nut which is visible in the picture.

PLATE VI