Mineral Guide > Gems > Cutting of Gems

Cutting of Gems

The condition in which gems are found in nature is rarely such as, according to the general notion of human kind, exhibits their greatest beauty. In the state of nature, the surfaces of gems are generally dull and lusterless; their shape is irregular, and their mass is permeated by flaws and imperfections. Moreover, the powers of reflection and refraction of light, which give gems their superior brilliancy and fire, can only be brought out in perfection when the stones are shaped with reference to their internal structure. Hence, from the earliest times, man has endeavored to increase the beauty of gems by bringing them to a condition of the highest possible polish and luster.

The progress of this art has been a gradual and slow one; but in its present development it affords an opportunity for the exercise of knowledge and skill of a high order. It is true that from time to time certain -art critics, among whom was Ruskin, have urged that gems in their native state are more beautiful than when cut, but such views overlook the obvious enhancing of the optical qualities of a gem by a proper cutting. The mere facetting of a stone may be, as these critics claim, an expression of a somewhat vulgar taste; but cutting a stone with reference to its optical structure applies an intelligent skill which can but prove enhancing to its natural beauty. Occasionally a diamond or ruby crystal is found of sufficient regularity of form and purity to make it available in its natural state for use as a gem; but ordinarily the art of the lapidary is needed to bring from precious stones an exhibition of their full beauties, and fit them for the highest purposes of ornament. On the other hand, there is a common notion as to the amount of improvement that can be made in a stone by cutting or facetting, which is generally a mistaken one. There is no stone so dull and lusterless that some one will not think that it would be beautiful if it could only be cut and polished. But as a matter of fact, cutting or polishing usually changes the appearance of a stone very little, and a stone which is not attractive in color and transparency before cutting is not likely to be after. The skilled lapidary, it is true, can select the most favorable parts of a mass for cutting, but more than this he cannot do; and much disappointment may be avoided if only those stones are cut which can be seen while in the rough to have the necessary desirable qualities.

The first effort on the part of man to improve upon the natural appearance of gems was confined to giving them a simple polish. At first only the natural surfaces were polished, but later the rough corners were rounded, and gradually the plan of giving them a symmetrical shape developed. To this day, however, the treatment of gems in the Orient is confined largely to rounding and polishing the stones, with little alteration of their natural shape. The Kohinoor diamond in the form in which it reached England is an illustration of the unsymmetrical shape which is allowed by Orientals to be retained by even their most costly gems. The appliances by which this work of polishing and cutting gems is still performed in the East are of the crudest kind, and show little advancement from the earliest types used.

Among Occidental peoples, the cutting of gems was early carried to a much higher point than among Orientals. By both Greeks and Romans gems were given a symmetrical form, and they carried to a high degree of perfection the art of cutting cameos and intaglios from them.

The different forms into which precious stones are cut at the present time may be arranged in two groups:

(1) those having plane surfaces ; and
(2) those having curved surfaces, although the two may be combined in the same cutting.

The different forms under these subdivisions may be grouped, following Church, thus:

1. Plane surface cuttings............Brilliant / Step or Trap / Mixed or Brilliant Top / Table / Rose
2. Curved surface cuttings.........Single cabochon / Double cabochon / Hollow cabochon / Tallow top

Of these cuttings, those of the first group are usually used for transparent stones, such as the diamond, emerald, and ruby; and those of the second for translucent and opaque gems, such as the opal, turquois, moonstone, cat's-eye, and the like. The garnet is cut in both ways, the cabochon-cut garnet being called a carbuncle.

The question as to which form of cutting should be used for any particular gem is one involving considerations of the mineral species and the peculiarities of each individual stone. On the one hand, it is desirable to avoid as little loss of the stone as possible; and on the other, to give it that shape and proportion which shall best bring out its luster, brilliancy, and color. Pale stones should, for instance, have greater depth than dark ones; the latter should be given more "spread" and less depth. A well-cut stone is worth considerably more than a poorly cut one, even if the latter has a greater weight. 'Often in cutting a stone one-half and even more of its mass may be removed, and yet the stone be improved thereby. The brilliancy of a stone is increased, others things being equal, the larger ti-,e number of facets which can be given it. The value of the stone must be taken into consideration in this connection, however. Stones of moderate value do not have their worth sufficiently increased by addition of numerous facets to warrant the expenditure of the extra time and labor that would be required to bring them to this condition. If a stone is strongly dichroic, as is iolite, for example, the cutting must be in such a direction as to bring out this quality in the highest degree. Similarly tourmaline, because of its dichroic properties, may make a dark and uninteresting stone if cut at right angles to the crystallographic axis, while if cut parallel to this axis it will make a brilliant appearance and show two colors. Such stones as moonstone, labradorite, tiger's-eye, and others, which show chatoyancy only in certain directions, must obviously be cut with reference to this feature. In transparent stones, the angle which the upper and lower facets make with each other should be a definite one, so as to reflect the light in the best possible manner.
Considering briefly and in order the forms of cutting above mentioned, we may note first the brilliant. The brilliant cut is said to have been invented by Cardinal Mazarin in his endeavors to introduce the art of diamond-cutting into France. It is now the form most commonly given diamonds and is employed for many other transparent stones as well.

The outline of the brilliant cut is usually circular. Brilliants are sometimes cut so as to have a nearly square outline, or again they may be made triangular, or again oval.

The second cutting to be noted is the trap or step cut. This is a favorite form of cutting for colored stones. It is a shallower cutting than the brilliant, and has a broader table. The outline is commonly oblong, in contrast to the more nearly circular form of the brilliant, although quadrilateral, hexagonal, and other outlines may be given. The rules of proportion are far less strict than those applied to the brilliant. The following form is a common one, however: Beginning with the table above, two sloping or step facets lead to the girdle, below which three to five or more sets, or zones of diminishing steps, extend to the culet. The latter has the general shape of the stone. The number of facets is often increased over the above with advantage. A common fault with the step cut comes from making the table too broad, since the internal reflections from the lower facets are best seen, as Church states, through the sloping bezils of the crown, not through the flat surface of the table. The mixed or brilliant top cut is a combination of the brilliant and step cut.

The table cut is a simpler cutting than either the step or brilliant. It consists simply of a table with beveled edges. It is an old form of cutting, and is generally superseded at the present day by forms with a greater number of facets. The rose cut has the crown facetted all over, the table of the brilliant being replaced by six triangular facets, and the other facets by eighteen triangular ones. The base is either made flat, or as a duplicate of the upper part, the latter cut giving what is known as the " double brilliant."

The rose cut is especially useful for small or flat diamonds, as by means of it well-cut gems can be made from pieces of "rough" which are too small or too thin to make brilliants.

Besides the brilliant and rose, which are standard cuttings for the diamond, there are several quaint and fanciful cuts which are now more or less in vogue. One of these is the " pendeloque," a sort of double rose cut, and the " briolette," also a double rose cut of a general pear shape. The outline of the stone may be varied also, so as to be triangular, hexagonal, or circular. A form of diamond cutting which is now being extensively advertised is that called the " twentieth century " cutting. This is a double rose cut with eighty planes, forty above and forty below. It is made up essentially of two cones placed base to base, both completely facetted with planes, eight of which meet around each apex.

The supposed superiority over the brilliant rests in the substitution of facets for the table and culet of the latter.
The curved cuttings given, to precious stones are all modifications of the form known as cabochon, the various shapes given being such as are best adapted to bring out the beauties of the individual stone which is to be cut. The hollow cabochon serves the purpose of lightening the color of dark stones, and affording a place for inserting a foil.

The manner in which the actual work of cutting and polishing gems is performed by the most advanced methods of the present day varies somewhat with the kind of stone.

Some stones naturally require a much harder abrasive than others, while different wheels and different polishing powders are suited to different gems. In general, the stone is reduced as nearly as possible to the desired shape by careful cleaving in the rough. If there is a natural cleavage much use can be made of this in bringing the stone to the desired shape; if not, the work cannot be carried far in this manner. Large stones, if not too hard, can be sawed to a desirable shape with diamond or carborundum saws. After having been shaped as nearly as possible by one of these methods, the rough stone is then soldered to a metal handle, or cemented to a stick by means of wax or other adhesive substance, and ground to a rounded symmetrical shape on a flat, revolving wheel, the abrasive used being applied by means of water or oil. The wheels used are generally either of iron or copper, though lead, tin, and even wooden wheels are employed. For all gems except the diamond, the cutting of which can be carried on only by means of diamond dust, emery or ground corundum is the abrasive generally used, although since the invention of carborundum this is employed quite extensively. After the stone has received a general rounding in this manner, the cutting of facets, one at a time, is begun. To maintain the exact angle at which each facet is to be cut, a clamp is provided above the wheel, in which is fastened the handle on which the gem is soldered. By this means the stone is held against the wheel at the desired angle until the facet is cut. For facetting cheap stones the handle of the gem is sometimes held in the hand; but while the work - can be done faster by this means it obviously cannot be performed so accurately. After the stone has received by grinding the proper number of facets, each of the size desired, the work of polishing must be performed. This is done in a similar way to the grinding, except that softer abrasives and softer wheels are used. Rouge, tripoli, and "putty powder" are the abrasives most commonly used for this purpose, they being applied dry or moist to wheels of leather, felt, or paper, against which the stone to be polished is held.

Owing to its superlative hardness, the cutting of the diamond must be performed by a somewhat different process than that of other stones. The facets upon a diamond are cut by rubbing together by hand two diamonds cemented upon sticks. After the facets have been outlined in this way they are ground and polished upon wheels to which diamond dust is applied, in a manner similar to that described for other gems.

The grinding and polishing of agates and other large stones are performed at Oberstein, Germany, on an extensive scale. The wheels for grinding turn vertically instead of horizontally, as is usually the arrangement when cutting small stones. They are made of sandstone, are about five feet in diameter, and often a foot in thickness. Their edges are often fluted in different shapes, so as to give different desired forms. The piece to be cut is held by the workman by hand against the wheel until it has received the desired shape. After being ground it is polished on a wheel of hardwood with tripoli, this part of the work being usually performed by women and children.