A Text-Book of Precious Stones for Jewelers and the Gem-Loving Public Part 12

FACETTED STONES. If, however, the stone is to be facetted in either the brilliant form, somewhat like the diamond, or step cut or otherwise facetted, it is cemented strongly onto a holder (much like the wooden part of a pen holder). The upper end of the holder is rested in one of a series of holes in what is called a "_ginpeg_" resting in the work-bench near a metal lap, and the stone is pressed upon the rapidly rotating surface of the lap, which is charged with diamond dust or carborundum, according to the hardness of the material to be facetted. A flat facet is thus ground upon the stone. By rotating the holder a series of facets, all in the same set, is produced. The holder is then changed to a new position on the ginpeg and another set of facets laid upon the stone. Thus as many as four or five tiers or sets of facets may be applied to one side, say the top of the stone. The latter is then removed from the holder and cemented to it again, this time with the bottom exposed, and several sets of facets applied.

The stone is now _cut_ but not _polished_. The facets are flat, but have a rough ground-gla.s.s like surface. The polis.h.i.+ng is usually done by workers who do not cut stones, but who do nothing but polish them. In small shops, however, the same lapidary performs all the parts of the work.

POLIs.h.i.+NG. The polis.h.i.+ng of stones, whether cabochon or facetted, is accomplished by the use of very finely powdered abrasives such as corundum powder, tripoli, pumice, putty powder, etc. Each gem material requires special treatment to obtain the best results. It is here that most of the trade secrets apply.

The troubles of the lapidary in getting the keen polish that is so much admired on fine gems are many. In general, the polis.h.i.+ng powder should not be quite as hard as the material to be polished, else it may grind rather than polish. The material should be used with water or oil to give it a creamy consistency. It should be backed by laps of different materials for different purposes. Thus, when backed by a fairly hard metal even tripoli, although much softer, will polish sapphire. On a lap of wood, tripoli would fail to polish hard materials, but would polish amethyst or other quartz gem. A change of speed of the lap, too, changes the effect of the polis.h.i.+ng material. I have seen a lapidary, who was having no success at polis.h.i.+ng an emerald, get very good results by using a stick as a brake and slowing down his lap.

The polis.h.i.+ng material must be of very uniform size, preferably water floated or oil floated, to give good results. The lap must be kept flat and true and the stone must be properly held, or the flatness of the facets, upon which brilliancy depends in part, will be destroyed during the polis.h.i.+ng.

The softer materials, such as opal, require treatment more like that accorded cut gla.s.s, and soft abrasive powders, such as pumice, suffice to polish them. Probably hardly two lapidaries would work exactly alike in their treatment of precious stones, and each guards his secrets, yet all use approximately similar general methods. Some have devised mechanical holders which permit the repeated cutting of stones to exactly the same angles, and that, too, with an accurate knowledge of the angles used. These angles can be definitely altered for different materials, according to their refractive indices. Other lapidaries produce very fine results by purely hand methods.

These details have been gone into to give an idea of the methods of the lapidary and of the many variations in method. In general, however, the _slitting_ or _cleaving_, the _rubbing down_ to shape, the _smoothing out_ of all scratches and the _facetting_ and _polis.h.i.+ng_ are done somewhat similarly by all lapidaries.

Having now had a glimpse of the methods of the lapidaries, let us briefly consider what const.i.tutes good "make" in stones other than diamond.

GOOD "MAKE" IN COLORED STONES. Brilliants, cut from materials having smaller refractive indices than diamond, (and this group includes nearly all stones other than diamonds) should have steeper back angles and higher tops than the best diamond brilliants have. A 35-degree top angle (the angle between the slope of the top and the plane of the girdle is called the top angle) and a 41-degree back angle being about ideal for diamond, other gem materials should have more nearly a 39-degree top angle and a 44-degree back angle to give the greatest possible brilliancy. However, in the case of colored gems such as ruby, sapphire, etc., where the value depends even more largely upon the color than upon the brilliancy, it is frequently necessary to cut the brilliant thicker or thinner than these proportions in order to deepen or to thin the color.

In general, the thicker a stone of a given spread the deeper the color will be. The color may also be deepened by giving to the stone a rounded contour, both above and below the girdle, and facetting it in steps instead of in the brilliant form. Increasing the number of steps also serves to slightly deepen the color, as a larger number of reflections is thus obtained within the material, the light thus has to travel a greater distance through the colored ma.s.s, and more of the light, of color other than that of the stone, is absorbed.

IMPROVING COLOR BY PROPER CUTTING. In addition to the color improvement that can be brought about by changing the shape of the cut stone there are a number of gem materials whose color varies very greatly in different directions, and this fact calls for skillful use in order to obtain the best possible results. Thus most tourmalines of deep color must be cut with the top or table, of the finished stone, on the _side_ of the prismatic crystal rather than at right angles to the axis of the prism. If cut the latter way they would be much too dense in color. On the other hand, most blue sapphires should be cut _across_ the prism axis rather than the way that tourmalines should be cut. To cut a sapphire with its table on the side of the prism would be likely to cause it to have a greenish cast because of the admixture of the unpleasing "ordinary ray" of yellowish tint with the blue of the stone as seen up and down the prism. Some Australian sapphires are of a p.r.o.nounced green when viewed across the axis of the crystal.

Rubies if cut, as was recommended for sapphires, give a very pure and very deep red color, but lack somewhat in the display of dichroism given by rubies that are cut with the table on the side of the crystal and parallel to its axis. Lapidaries need to know and to make use of such optical relations as these and jewelers might well inform themselves in such matters, especially if they have, or hope to acquire, trade in very fine colored stones.

EFFECT OF SHAPE ON BRILLIANCY. In actual practice it is common to find colored stones poorly cut for brilliancy, especially central brilliancy, and that, too, without the excuse of sacrifice of brilliancy in order to improve color. The fault is usually due to too great a desire to save size and weight. Frequently a stone would have greater value if properly cut, even at the expense of some size and weight. When stones are cut too shallow, as is frequently the case, they are sure to leak light in the center and they are thus weak and less brilliant there than they would be if made smaller in diameter and with steeper back slopes approximating 44 degrees.

Round stones, if their angles are correct, are more brilliant than stones of other contour such as square or cus.h.i.+on shape, or navette or heart shape. It can readily be seen that such odd-shaped stones can hardly have the same top and back angles at every part of their circ.u.mference. If the angle from a corner of a square stone is correct then the angle from the middle of one side is obviously a little different. Small differences of angle make considerable differences in the brilliancy of cut stones. The prevailing tendency to cut nearly all diamonds round depends largely upon the above facts. In the case of colored stones, however, the added attractiveness which comes with odd or different contour more than makes up for the slight loss of brilliancy that may attend upon the shape selected. Such shapes as lend themselves to special designs in mountings also justify any little loss in brilliancy that accompanies the change in shape, provided the proportions retained give a considerable amount of total reflection within the stone and thus light up most of the stone as seen from the front.

The test of the "make" of a color stone is its appearance. If it lights up well over most of its surface and if the color is right, one should not criticize the "make" as one would be justified in doing in the case of a diamond. If, however, the effect is less attractive it would many times be advisable to measure the angles of the stone, or its thickness and spread as compared with similar measurements on a stone of fine appearance. Frequently one will thus find the reason for the failure of the stone to perform as it might, and recutting should be resorted to in such cases in order to get a smaller but more beautiful and hence more valuable stone.

LESSON XXIV

FORMS GIVEN TO PRECIOUS STONES

While precious stones are cut to many different forms, there are, nevertheless, but a few general types of cutting. These may be cla.s.sified as follows: First, the "_cabochon_" (Fig. 11) type of cutting; second, the old "_rose_" (Fig. 12) type of cutting; third, the _brilliant_ (Fig. 13); fourth, the _step cutting_ (Fig. 14).

CABOCHONS. Of these the first, or _cabochon_ cutting, is probably the most ancient. The term comes from a French word signifying a bald pate (caboche, from Latin cabo, a head). The usual round cabochon cut closely resembles the top of a head in shape. Cabochon cut stones usually have a flat base, but sometimes a slightly convex base is used, especially in opals and in moonstones, and some stones of very dense color are cut with a concave base to thin them and thus to reduce their color. The contour of the base may be round, or oval, or square, or cus.h.i.+on shape, or heart shape or of any regular form. The top is always smooth and rounding and unfacetted. The relation of the height or thickness to the length or width may be varied to suit the size and shape of the rough piece or to suit one's ideas of symmetry, provided the material be an opaque one, such as turquoise or lapis lazuli. If, however, the material is transparent the best results in the way of the return of light to the front, and hence in the display of the color of the material, are had if the thickness is about one half the spread.

[Ill.u.s.tration: FIG. 11.--CABOCHON CUTTING.]

This relation depends upon the refractive index of the material, but as most color stones are of somewhat similar refractive indices, the above proportions are sufficiently accurate for all. The object in view is the securing of total reflection of as much light as possible from the flat polished back of the stone. Cabochon stones are sometimes set over foil or on polished gold to increase the reflection of light.

The path of a ray of light through a cabochon cut stone is closely similar to that through a rose cut diamond [see cut (c) of Fig. 12 for the latter.] Like the rose cut, the cabochon cut does not give much brilliancy as compared to the brilliant cut. Cabochon cut stones, however, have a quiet beauty of color which commends them to people of quiet taste, and even fine rubies, sapphires, and emeralds are increasingly cut cabochon to satisfy the growing demand for fine taste in jewels. The East Indian has all along preferred the cabochon cut for color stones, but possibly his motives have not been unmixed, as the cabochon cut saves a greater proportion of the weight of the rough stone than the more modern types of cutting.

Garnets, more than other stones, have been used in the cabochon cut, and when in that form are usually known as _carbuncles_ (from carbunculus, a glowing coal). Any other fiery red stone might equally well be styled a carbuncle, especially if cabochon cut.

[Ill.u.s.tration: FIG. 12.--ROSE CUTTING.]

Scientific rubies look very well in the cabochon cut.

Fig. 11 shows in (a) and (b) the front and top of the usual round cabochon. Cut (c) of the same figure gives the front elevation of a cabochon which will light up better than the usual round-topped design.

In the round-topped type the central part of the top is so nearly parallel to the back that light can pa.s.s right through as through a window pane. If the sloping sides are brought up to a blunt point, as in cut (c) there is very much less loss of light and greater beauty results. The East Indian cabochons are frequently cut in a fas.h.i.+on resembling that suggested.

[Ill.u.s.tration: FIG. 13.--BRILLIANT CUTTING.]

ROSE CUT STONES. It was natural that the earliest cut stones should have the simple rounded lines of the cabochon cutting, for the first thing that would occur to the primitive worker who aspired to improve upon nature's product, would be the rubbing down of sharp edges and the polis.h.i.+ng of the whole surface of the stone. Perhaps the next improvement was the polis.h.i.+ng of flat facets upon the rounded top of a cabochon stone. This process gives us the ancient type of cutting known as the _rose_ cut. The drawings (a) and (b) of Fig. 12 show the front elevation and the top and (c) shows the path of a ray of light through a "rose." It will be noted that the general shape resembles that of a round cabochon, but twenty-four triangular facets have been formed upon the top. The well-proportioned rose has a thickness about one half as great as its diameter. Diamonds were formerly cut chiefly in the rose form, especially in the days of the East Indian mines, and even in the early part of the nineteenth century many people preferred finely made roses to the thick, clumsy brilliants of that day. To-day only very small pieces of diamond are cut to "roses." As the material so used frequently results from the cleaving of larger diamonds, the public has come to know these tiny roses as "chips."

The best roses have twenty-four regular facets but small ones frequently receive only twelve, and those are seldom regular in shape and in arrangement. Such roses serve well enough for encrusting watch cases and for similar work, as the flat base of the stone can be set in thin metal without difficulty. About the only gem other than diamond that is now cut to the rose form is garnet. Large numbers of small Bohemian garnets are cut to crude rose form for use in cl.u.s.ter work.

[Ill.u.s.tration: FIG. 14.--STEP CUTTING.]

THE BRILLIANT CUT, as its name implies, gives the most complete return of light of any of the forms of cutting. The theory of the brilliant has already been discussed (Lesson XXII. in connection with the cutting of diamond). The shape of the brilliant is too well known to require much description. Most brilliants to-day are cut practically round and the form is that of two truncated cones placed base to base. The upper cone is truncated more than the lower, thus forming the large, flat top facet known as the _table_ of the stone [A, Fig. 13, cut (a)]. The truncating of the lower cone forms the tiny facet known as the culet, which lies opposite to the table and is parallel to the latter [see B, Fig. 13, cut (a)]. The edge of meeting of the two cones is the _girdle_ of the brilliant [CD in cut (a), Fig. 13]. The sloping surface of the upper cone is facetted with thirty-two facets in the full cut brilliant, while the lower cone receives twenty-four.

Small stones sometimes receive fewer facets, to lessen the cost and difficulty of cutting, but by paying sufficient for them full cut brilliants as small as one hundred to the carat may be had. Cut (b) of Fig. 13 shows the proper arrangement of the top facets and cut (c) that of the bottom facets.

When cutting colored stones in the brilliant cut, especially if the material is very costly and its color in need of being darkened or lightened, the lapidary frequently takes liberties with the regular arrangement and proportions depicted in the cuts.

STEP CUTTING. The only remaining type of cutting that is in very general use is the _step cut_ (sometimes known as trap cut). Fig. 14, (a), (b), and (c), shows the front elevation, the top and the back of a square antique step cut stone. The contour may be round or completely square or oblong or of some other shape, just as a brilliant may have any of these contours. The distinctive feature of the step cutting is the several series of parallel-edged quadrangular facets above and below the girdle and the generally rounding character of its cross section. This plump, rounding character permits the saving of weight of the rough material, and by ma.s.sing the color gives usually a greater depth of color than a brilliant of the same spread would have if cut from similar material.

While probably never quite as snappy and brilliant as the regular brilliant cut, a well-proportioned step cut stone can be very brilliant.

Many fine diamonds have recently been cut in steps for use in exclusive jewelry.

THE MIXED CUT. The ruby and the emerald are never better in color than when in the full step cut, although rubies are frequently cut in what is known as the _mixed_ cut, consisting of a brilliant cut top and a step cut back. Sapphires and many other colored stones are commonly cut in the mixed cut. Recently it has become common to polish the tops of colored stones with a smooth unfacetted, slightly convex surface, the back being facetted in either the brilliant or the step arrangement.

Such stones are said to have a "_buffed top_." They are less expensive to cut than fully facetted stones and do not have the snappy brilliancy of the latter. They do, however, show off the intrinsic color of the material very well.

LESSON XXV

IMITATIONS OF PRECIOUS STONES

"PASTE" GEMS. Large volumes have been written on paste jewels, especially on antique pastes. Contrary to a prevailing belief, the paste gem is not a recent invention. People frequently say when told that their gems are false, "But it is a very old piece, it must be genuine."

The great age of a jewel should rather lead to suspicion that it was not genuine than give confidence that a true gem was a.s.sured. The Egyptians and Romans were skillful makers of gla.s.s of the sort used in imitating gems and some of the old pastes were very hard or else have become so with age.

Gla.s.s of one variety or another makes the most convincing sort of imitation precious stones. The term "paste" as applied to gla.s.s imitations is said to come from the Italian _pasta_ meaning dough, and it suggests the softness of the material. Most pastes are mainly lead gla.s.s. As we saw in Lesson XVIII., on the chemical composition of the gems, many of them are silicates of metals. Now gla.s.ses are also silicates of various metals, but unlike gem minerals the gla.s.ses are not crystalline but rather amorphous, that is, without definite geometric form or definite internal arrangement.

The optical properties of the various gla.s.ses vary chiefly with their densities, and the denser the material the higher the refractive index and the greater the dispersion. Thus to get the best results in imitation stones they should be made of very heavy gla.s.s. The dense flint gla.s.s (chiefly a silicate of pota.s.sium and lead) which is used for cut gla.s.s ware ill.u.s.trates admirably the optical properties of the heavy gla.s.ses. By using even more lead a still denser gla.s.s may be had, with even a greater brilliancy.

Unfortunately the addition of lead or other heavy metals (such as thallium) makes the product very soft and also very subject to attack by gases such as are always present in the atmosphere of cities. This softness causes the stones to scratch readily so that when worn they soon lose their polish and with the loss of polish they lose their beauty. The attack of the gases before mentioned darkens the surfaces of the imitation and further dulls it. When fresh and new a well cut piece of colorless paste has a snap and fire that approaches that of diamond.

The surface l.u.s.ter is not adamantine, however, and the edges of the facets cannot be polished so sharply as those on a diamond. Moreover the refractive index, while high, is never so high as in a diamond and hence the brilliant cannot be so shaped as to secure the amount of total reflection given by a well-made diamond. Hence, the paste brilliant, while quite satisfying as seen from squarely in front, is weak and dark in the center as seen when tilted to one side. By these differences the trained eye can detect paste imitations of diamond at a glance without recourse to tests of specific gravity, hardness, etc.

Pastes, being amorphous, are singly refracting, as is diamond. This fact helps the appearance of the paste brilliant, for light does not divide within it to become weakened in power. This singleness of refraction, however, betrays the paste imitation when it is colored to resemble ruby, sapphire or emerald, all of which are doubly refracting.

The color is imparted to pastes by the addition, during their manufacture, of various metallic oxides in small proportions. Thus cobalt gives a blue color, copper or chromium green, copper or gold give red (under proper treatment) and manganese gives purple. By experiment the makers of pastes have become very skillful in imitating the color of almost any precious stone. Fine paste emeralds may look better than inferior genuine emeralds.