CUTTING OF PRECIOUS STONES OTHER THAN THE DIAMOND
All the other precious stones are less hard than the diamond, and they display besides the greatest difference from each other in this respect. Accordingly, through the processes followed in cutting them are not very dissimilar outwardly to those by which the diamond is cut and polished, yet the materials made use of are very different. The wheels have the same form and are set up in the same way, but they are made of much softer materials, and the powders with which are covered are much less hard than diamond-dust.
Disks of lead, tin, or sometimes zinc, copper, and hard wood, are what the ordinary lapidaries use, and instead of diamond-dust they employ emery (a substance consisting chiefly of alumina), tripoli or rottenstone (silica), tin-putty (bioxide of tin), and English red (anhydrous peroxide of iron). Different wheels and polishing substances are used according to the kind of stone. The greater part of colourless precious stones are cut with the leaden wheel, and with rotten-stone well moistened. This serves to give the first polish to all precious stones in which silica is the principal element-agates, jaspers, hyacinths, &c.
The two styles most employed are the step-cut and the smooth-cut or cabochon. When the latter is very flat it is called the "tallow-drop". Each of these may be round or oval, elongated or square.
The cabochon is plane, convex or concave on its inferior side. In the latter case it is the double cabochon.
Concave cabochons are employed for stones moderately transparent, and this disposition tends to facilitate a more easy transmission of light. Garnets of a certain size are often cut in this form; and this cutting is used especially for the adularia, the cat's-eye, the hydrophane, and, above all, the opal.
It serves better than any other form to display the special beauties of these different stones.
The stones cut in step or pavilion form are generally not very thick, and there are usually more steps or degrees on the lower than the upper side; as on the upper a large table in the centre is generally reserved. Figs. 121 and 122 represent forms given to a great number of coloured stones, especially the emerald and the oriental aquamarine.
There are other forms, in which the stones cut in circles or ovals have a large table on the upper surface, surrounded with facets, which are either triangular or triangular and quadrangular both. In this case the lower face is covered with quadrilateral facets, and has a very small table in the centre.
Rubies and sapphires are frequently cut very much like the diamond, with this difference, that there is less thickness given to their upper part.
MOUNTING OF PRECIOUS STONES
Precious stones, which enter into jewelry in a thousand ways, are rarely seen simply pierced and suspended as a drop; on the contrary, they are nearly always elaborately mounted in silver or gold. At the present day gold is used almost exclusively for the setting both of colourless and coloured stones; but silver is considered more artistic for the former as it preserves their limpidity and brilliancy, and even lends them additional splendour, as gold does to the coloured stones.
There are two modes of mounting precious stones--one, which leaves only the upper part visible, called the close-setting; and the other, leaving the stone uncovered both above and below, called the open-setting.
A modification of the open-setting, called the knife-edge setting, leaving the edge of the stone clear, is used with beautiful effect for diamonds.
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Precious Stones Guide Vol 2
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