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The Garnet-and-Glass Doublet
Historically, this was the most common doublet. It was made to imitate transparent gemstones of every color, for it was by far the most versatile substitute available prior to synthetics. It is still used today. It has always been more suitable than glass, because the garnet crown is much more durable and its high luster adds materially to the attractiveness of the stone. Excellent color imitations of topaz, sapphire, emerald, ruby and amethyst have been made in the garnet-and-glass combination, as well as very good substitutes for garnet itself.
The basic method of making garnet-and-glass doublets utilizes a steel plate approximately one inch in thickness through which several one-half to one inch holes have been bored. Garnet (usually almandite) is placed in each hole to rest on the fire clay. The holes are then filled with a powder that fuses into glass when the clay and plate are placed in an oven. After cooling, the plugs are removed from the plate and stones fashioned with the garnet layer uppermost. The reason that garnet, rather than some other hard mineral, has been used is that it fuses to glass without cracking and the fused portions can be fashioned without apparent undercutting of the glass. To speed the fashioning process and to keep costs at a minimum, these doublets are cut rapidly and little attention is given to the alignment of the two portions.
As a result, the garnet cap rarely constitutes the total crown of the stone; more often, it is an irregular section covering the table and a portion of the top part of the crown (Figure 1). It may be seen from this that the garnet portion in small stones may extend part way down the pavilion, although this is uncommon. Usually, only a small section of the bezel area beyond the table is covered by the garnet.
To one who is not familiar with garnet-and-glass doublets, it is difficult to believe that the garnet cap has to effect on the color of the final product. Those that were once made to imitate diamond were completely colorless, and the red flashes that one would expect to see in a sapphire-blue imitation comparable to the flashes displayed by dark-blue synthetic spinel) are completely lacking.
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Emerald Substitutes
The emerald doublet or triplet, as indicated previously, is still an important substitute. Probably the most commonly encountered one today is either a quartz-and-quartz triplet with a green cement layer, or two pieces of colorless synthetic spinel joined with the same cement (Figure 2). Flawed rock crystal is often used to imitate the flaws in emerald, and the spinel may be quench crackled to produce the same effect. Occasionally, stones consisting of two parts of pale aquamarine or colorless beryl joined by a green cement are encountered, but much less frequently.
If there various triplets are well made and given reasonable care, they will endure for an extended period of time. There appears to be a tendency for some of the older stones to separate along the joining plane, but most modern stones are quite durable. At one time, the only satisfactory cement was Canada balsam today, however, plastic adhesives with astonishing properties are available that, unlike Canada balsam, do not alter in color nor deteriorate with age. Of course, garnet-and-green-glass doublets are still used as emerald imitations however, those that are encountered are usually in old jewelry pieces are were taken from such pieces. Occasionally, parcels of “fresh" stone s are released to the trade from some manufacturer’s old stock.
Methods of manufacturing triplets vary, but if the hardness of the two portions varies considerably, the piece s are often fashioned separately and then cemented. This can sometimes be proved by observing the girdle edge and noting slight irregularities, which would not have been left if the cementing had been done first and the stone polished a s a whole afterward, In fact, even if they are made of the same materials, they are usually cemented after manufacture of the two parts, and any misfitting is corrected somewhat by a quick trimming on the polishing wheel. Even when cementing is done before finishing the stone, the surfaces to be cemented are polished. Other facets may or may not be rough ground before the cementing is accomplished.
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Opal Substitutes
Gem opal is rare and expensive. Gem-quality material frequently occurs in very thin seams. Since the mineral is very fragile, it is impossible to fashion stones that are sufficiently durable for jewelry use from these thin pieces. Therefore, in order to salvage all valuable material and to produce stones thick enough to resist breakage, the cutter cements the thin pieces to a backing of common opal, glass, chalcedony, or even pieces of opal matrix (Figure 3). The two portions are usually joined together with a black, pitch like cement, which provides two advantages: First, since darker opals are usually more valuable than the light-colored stones, it enhances the impression of expensiveness; secondly, because it is soft and resilient, it provides a cushion for shocks and reduces the possibility of damage to the stone. A colorless cement may be used with a backing material of black chalcedony, so that the same appearance is created.
Usually, no effort is made to conceal the fact that the stone is a doublet. It is obvious that the purpose for making it was to utilize the valuable material and make it saleable, rather than to attempt to defraud. Of course, this is not always true, for sometimes a type of setting is used that intentionally conceals the joining plane.
A very poor imitation of opal is made by using a thin piece of mother-of-pearl sandwiched between layers of glass. It does not bear enough resemblance to opal to be of any significance.
There are two variations of the opal doublet that are also used to strengthen an otherwise thin piece of material. One of these is made by fitting the sepal into a depression in a piece of piece of dyed black chalcedony (Figure 4). The other type consists of cementing small fragments of opal into a similar depression to form a mosaic (Figure 5); final polishing reduces the individual pieces to one plane. Both methods produce the effect of opal framed by black.
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Star Sapphire Substitutes
Another kind of manufactured gem utilizing a backing material is the inexpensive star sapphire substitute. It is made by affixing a blue mirror to the flat back of a star rose quartz cabochon, which serves the dual purpose of strengthening the star and producing a color similar to that of star sapphire. Since star quartz, particularly with a mirror backing, provides fairly noticeable asterism, it was of some importance commercial before the introduction of synthetic star sapphires. When the latter became available, the incentive to continue manufacturing and improving the quartz imitations was removed; as a result, they are much less important than formerly.
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Diamond Substitutes
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forms just the crown of what appears to be a very large stone. It is then mounted, in a setting that conceals the back completely. Actually, the crown is nothing more than a cap over a cavity in the mounting. The illusion of back facets is created by molding a pattern into a highly reflective metal insert placed below the stone. The diamond cap is set flush against the top of the metal insert, to make an air-tight bond. Sometimes, the impression that a single stone into an opening at the base of the mounting, thus simulating the appearance of the pavilion of a very large stone (Figure 6). Perhaps the most important foil backed stone commercially is the colorless lead-glass brilliant-cut imitation of diamond called rhinestone. Originally, the term was applied to colorless quartz pebbles from the Rhine River that were cut to imitate diamond and foiled to increase brilliancy. When highly dispersive glass became available, it was substituted for the quartz.
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Miscellaneous Assembled Stones
On occasion, various types of assembled stones appear in the jewelry trade for only a short time. An example was a doublet with a synthetic corundum crown and a synthetic rutile base. The crown, of course, was considerably more durable than the pavilion and seemed to increase brilliancy durable than the pavilion and seemed to increase brilliancy somewhat, by permitting more light to escape from the top of the stone. The color of many pale-colored natural stones may be deepened and improved, with an apparent increase in value, by adding a colored cement or liquid between the back of the stone and its mounting. This is particularly true of emeralds, rubies and pargarnets in old jewelry. A reflective mirror alone accomplishes the desired effect on garnets. Al though doublets made with colored cement (triplets) are used most frequently to imitate emerald, those utilizing other colors of cement is encountered also. Sapphire-blue imitations are the most prevalent, although yellow, purple and red are seen occasionally as well. These doublets are most frequently made of either quartz or synthetic spinel.
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in Italy, is known as Florentine mosaic work. It is made by insetting bits of marble of various colors into a black marble base in a manner that creates the effect of a miniature painting of a nature scene, flowers or other objects of beauty. Other materials also have been used for this kind of work. Perhaps more common than marble are the mosaics made in Naples that consists of tiny fragments of stained bone inlaid into recessed areas in black glass. The pieces of bone are placed end up and pressed closely together to form a picture. Frequently, the design chosen is a Roman ruin, a countryside scene, Mt. Vesuvius or various animal forms. Although the workmanship required seems just as great using inexpensive bone and glass as in using carefully selected marble, the mosaic markers apparently find it expedient to use the cheaper materials. Well-executed mosaics for jewelry use are not commonly encountered.
