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GEMS

Gem Cutting

APPLICATION AS JEWELS

The use of precious stones as gems is much more extensive and varied than for any other purpose. In their rough state they have not, as a rule, a pleasing appearance, and therefore are unsuitable for this purpose; it is only after cutting and polishing that their beauty appears in all its fullness.

The process of cutting aims at giving each stone such a form as will best display its natural lustre and beauty. Thus the form in one case may be rounded, in another bounded by small faces or facets, the latter being very frequently used. The various modes of cutting in vogue at the present day, each of which is best suited to the idiosyncrasy of the particular stone to which it is applied, are the results of centuries of trial and observation on the part of gem-cutters. Thus the form in which transparent stones are cut differs from that best suited to opaque stones; and in the same way, the form in which dark-coloured stones are cut differs from that given to lighter or colourless specimens. The appearance of each would suffer if it were given any other than its own appropriate form.

The amount of refraction and dispersion exercised upon light by a transparent stone greatly affects its appearance, as has been shown in the case of diamond. It has also been shown that to obtain a maximum effect, the greater part of the light, which enters by the front facets of a cut stone must be reflected from the back facets, and must again pass out by the front facets. Since the path of a ray of light in a stone varies with the refractive index of the stone, and this character is different in different stones, it follows that the form of cutting must be adapted to the requirements of each particular case. It is thus the task of the gem-cutter to give to each stone that form which is calculated to bring out and display its beauties to the greatest possible advantage, and which, at the same time, involves the least possible waste of valuable material.

Gem-cutters, by prolonged experience, have arrived at certain empirical rules which are always applied, and which are modified to meet particular cases. In colourless stones, for example, there must be a fixed proportion between their breadth and their thickness, and there should be also certain relations between the shape of the back and that of the front of the stone.

Too great depth in a cut stone is as inimical to the full effect of its beauty as is too great shallowness. In the one case a stone is said to be thick or "lumpy", and in the other thin or "spread". Of two similar stones, one of which errs on the side of too great depth, and the other on that of too great shallowness, the latter is to be preferred. The facets at the back of the stone must occupy a certain position relative to the front facets; otherwise the light entering by these will not be totally reflected from the back. The same rules apply also to coloured stones. In this case the depth the stones are cut is important, and this must vary with their intensity of colour. A deeply coloured stone if too thick will appear dark or almost black, while a pale coloured stone will not exhibit a sufficient depth of colour unless it is cut of some thickness.

So long as the mutual relations of the facets of a cut stone are correct, the direction these take relative to the faces of the natural crystal is in most cases immaterial. In a few special eases, however, the directions of the cut facets must bear a definite relation to certain crystallographic directions in the stone. Thus the special colour effects of labradorite, moonstone, etc, are only manifest in certain directions; if cut in other directions, the beautiful effects for which these stones are prized would be lost. This is also true in the case of dichroïc stones, which, as we have already seen, vary in colour in different directions. Other cases of the same kind will be mentioned with the special description of each precious stone.

In the cutting of any given rough stone, not only must it receive the form best calculated to display its special beauties, but the facets must be cut in such positions as to involve the least possible waste of material, thus obtaining the largest possible size for the cut stone. In considering the positions in which the facets are to be cut relative to the boundaries of the rough stone, there are still other points, which may require attention. Thus the rough stone may have a flaw, and in this case the facets should be so placed that the faulty material will be cut away altogether, or, at least, so located in the cut stone that the beauty of the latter is impaired as little as possible.

With rough material containing flaws, a question will often arise as to whether, in the cut stone, size should be sacrificed to beauty, or vice versa. European gem-cutters are generally unanimous in the opinion that such a specimen should be cut so as to attain the highest possible degree of perfection and beauty even if this should involve considerable loss of material. A small stone, all the beautiful features of which are displayed to their full advantage, is more highly prized than a larger stone, the beauty of which is less perfectly developed. In every rough stone, the aim of the gem-cutter is to obtain a cut stone of the largest possible weight combined with the greatest possible beauty, since, the latter condition being fulfilled, the price obtained for the cut stone varies with its weight. The earnings of a cutter of precious stones depend largely upon his skill in treating each stone so as to obtain the greatest effect with the least waste of material.

These principles have not always been followed, for in earlier times the aim in gem cutting was to reduce the size and weight of the stone as little as possible. This is the case even at the present day in India and the East generally, as well as in various remote parts of the world where precious stones are found. Stones so cut have their facets very irregularly grouped, and consequently much of their beauty is undeveloped. Such stones are unfit for use in European jewellery, and are frequently re-cut according to modern principles; the increased beauty of their appearance so obtained more than compensates for the loss of the material cut away.

We now pass to the consideration of the various shapes and forms in which precious stones are cut with a view to their use in jewellery.

A. FORMS OF CUTTING

The various forms of cutting which have been found by experience to be most effective for gems and which are at present exclusively used, at least for valuable stones, fall naturally into two groups. The one includes all forms having facets; the other embraces forms of a rounded or cabochon shape. All faceted forms may be referred to one or other of four types according to the number and arrangement of the facets; forms intermediate between these types may also be met with.

The facets of a cut stone may be more or less uniformly distributed on all sides, or, again, they may be all located on one side, the other side being occupied by a single large face.


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Rafal Swiecki, geological engineer email contact

This document is in the public domain.

March, 2011