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 classified 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 cushion 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.
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.
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 pass 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 fashion resembling that suggested.
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 polishing of the whole surface of the stone. Perhaps the next improvement was the polishing 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 cluster work.
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 massing 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 assured. The Egyptians and Romans were skillful makers of glass of the sort used in imitating gems and some of the old pastes were very hard or else have become so with age.
Glass of one variety or another makes the most convincing sort of imitation precious stones. The term "paste" as applied to glass imitations is said to come from the Italian pasta meaning dough, and it suggests the softness of the material. Most pastes are mainly lead glass. As we saw in [Lesson XVIII.], on the chemical composition of the gems, many of them are silicates of metals. Now glasses are also silicates of various metals, but unlike gem minerals the glasses are not crystalline but rather amorphous, that is, without definite geometric form or definite internal arrangement.
The optical properties of the various glasses 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 glass. The dense flint glass (chiefly a silicate of potassium and lead) which is used for cut glass ware illustrates admirably the optical properties of the heavy glasses. By using even more lead a still denser glass 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 luster 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.
As pastes are singly refracting and hence lack dichroism, the pleasing variety of color of the true ruby cannot be had in a paste imitation, but the public is not critical enough to notice this lack. The expert would, however, note it and could detect the imitation by that difference as well as by the lack of double refraction. The use of direct sunlight and a white card as already explained in the lesson on double refraction ([Lesson III.]) will serve to expose the singleness of refraction of paste imitations. Spinels and garnets are about the only true gems (except diamond) that are single refracting. Any other color stone should show double refraction when tested by the sunlight-card method. The file test will also expose any paste imitation as all the very brilliant pastes are fairly soft.
Doublets. To give better wearing quality to paste imitations the doublet was devised. This name is used because the product is in two parts, a lower or back portion of paste and an upper or top portion of some cheap but hard genuine stone. Garnet is probably used for this purpose to a greater extent than any other material, although quartz or colorless topaz will do very well.
The usual arrangement of the parts can be seen in [Fig. 15], the garnet covering only a part of the upper surface, namely the table part and a small portion of the sloping surface of the top. In high class doublets the hard mineral covers the paste to the girdle. (See [Fig. 16].) The color of the garnet does not interfere seriously with that of the paste.
Fig. 15. ONE FORM OF CHEAP DOUBLET.
If a "diamond" doublet is desired the slice of garnet is made nearly as thin as paper and it covers only the table of the brilliant. It is thus practically colorless. A thin slice of red garnet over a green background is not noticeable, as all the red is absorbed in passing through the green material beneath. With a blue base, the red upper layer may give a very slight purple effect. With yellow a slight orange tint results and of course with a red back no perceptible difference would result.
Fig. 16. ANOTHER FORM OF DOUBLET.
The two materials are cemented together, by means of a transparent waterproof cement. The triplet has already been described in [Lesson XII]. It is even better than the doublet and more difficult to detect. Both the file test and the sunlight-card test serve to detect doublets, as well as paste imitations, except that in the file test with the fully protected doublet the back of the stone must be tested with the file, as the girdle and top are of hard material.
In the sunlight-card test of a doublet (the refraction of garnet being single like that of glass), single images of the facets will be had on the card when the sunlight is reflected onto it. A reflection of the lower or inner surface of the garnet top can be seen also and this serves to still further identify a doublet or a triplet. The appearance of this reflection is much like that received on the card from the top of the table. It is larger than the reflections of the smaller facets and is but little colored.
Tests for Doublets. A trained eye can also detect a doublet or a triplet by noting the difference in the character of the surface luster of the garnet part and of the glass part. Garnet takes a keener and more resinous luster than glass. By tipping the doublet so that light is reflected to the eye from the sloping top surface, one can see at once where the garnet leaves off and the glass begins. Even through a show window one can tell a doublet in this way although here it is necessary to move oneself, instead of the stone, until a proper position is obtained to get a reflection from the top slope of the doublet.
If the garnet covers the whole top of the imitation then it is not possible to get so direct a comparison, but even here one can look first at the top surface and then at the back and thus compare the luster. It is also well to closely examine with a lens the region of the girdle, to see if any evidence of the joining of two materials can be seen. Frequently the lapidary bevels the edge so as to bring the line of junction between real and false material at the sharp edge of the bevel. Boiling a doublet in alcohol or chloroform will frequently dissolve the cement and separate the parts.
The dichroscope also serves to detect the false character of doublets and paste imitations, as neither shows dichroism. As rubies, emeralds, sapphires, and in fact most colored stones of value, show distinct dichroism, this test is a sure one against these imitations.
Triplets and doublets too may be exposed by dipping them sidewise into oil, thus removing the prismatic refraction almost completely, as the oil has about the same refractive index as the stone. One can then look directly through glass and garnet, or other topping material, separately, and each material then shows its proper color. Thus zones of color appear in a doublet or triplet when under the oil. A real gem would appear almost uniform in color under these conditions.
Round gas bubbles can frequently be found in paste, and hence in the paste part of a doublet. Also, the natural flaws of the real stone are never found in paste, but may be present in the real stone part of a doublet or a triplet. Some imitation emeralds on the market, however, have been made in a way to counterfeit the flaws and faults generally found in this stone.
Altered Stones. In addition to the out and out imitations made of paste, and the doublets, there are numerous imitations current in the trade that are made by staining or by otherwise altering the color of some genuine but inexpensive gem material.
For example, large quantities of somewhat porous chalcedony from Brazil are stained and sold in imitation of natural agate or sard or other stones. In many cases the staining is superficial, so that the stone has to be shaped before it is stained, then stained and polished.
Large quantities of slightly crackled quartz are stained to resemble lapis lazuli, and sold, usually with the title "Swiss Lapis." A file test will reveal the character of this imitation, as it is harder than a file, while true lapis is softer. The color too is never of so fine a blue as that of fine lapis. It has a Prussian blue effect.
Turquoises of inferior color are also sometimes stained to improve them. A better product is made artificially.
Opals are sometimes impregnated with organic matter, which is then charred, perhaps with sulphuric acid, thus giving them somewhat the appearance of black opal.
Opals are also imitated by adding oxide of tin to glass, thus imparting a slight milkiness to it. The imitation is then shaped from this glass by molding, and the back of the cabochon is given an irregular surface, which may be set over tinsel to give the effect of "fire."
Pale stones are frequently mounted over foil, or in enameled or stained settings and thus their color is seemingly improved.
Diamonds of poor color are occasionally "painted"; often the back of the brilliant is treated with a violet dyestuff, which even in so small an amount that it is difficult to detect, will neutralize the yellow of the stone and make it appear to be of a fine blue-white color. The "painting" is, of course, not permanent, so that such treatment of a diamond with a view to selling it is fraudulent. The painted stone may be detected by washing it with alcohol, when the dye will be removed and the off-color will become apparent. If the stone is unset one can see with a lens a wavery metallic appearance on the surfaces that have been "painted." This effect is due to the action of the very thin film of dye upon the light that falls upon it.
Besides the staining of genuine materials, they are sometimes altered in color by heat treatment, and this topic will be discussed in the next lesson.