Synthetic spinel, was first produced accidentally during an attempt to make a satisfactory blue synthetic sapphire by the Verneuil process. The addition of cobalt oxide, which seemed a logical source for the blue sapphire, was found to give a very uneven coloration. As a result, an attempt was made to substitute for the cobalt oxides tinctorial agents, but without marked success. Then it was decided that magnesium oxide might act as a flux when added to the aluminum oxide and distribute evenly the color imparted by the cobalt oxide. The color was very even, but a study of the characteristics of the resulting boules showed that they were squarish in outline (Figure 5). Further investigation proved the material to be not sapphire but a magnesium aluminate with the structure of spinel, rather than that of corundum.
In contrast to the other synthetics, which are virtually identical in
composition to their natural counterparts, most commercially produced
synthetic spinel is much lower in magnesium-oxide content. In fact,
the usual synthetic spinel is not made with the natural's ratio of one
Al2O3 to one MgO, but rather, it is made usually in the proportion of
2-1/2 Al2O3 to one MgO. It should be noted that it is the difference
in the chemistry of the synthetic, that is the extra alumina (Al2O3) which causes the anomalous double refraction we see under the polariscope. This excess alumina alters the physical and optical properties of the spinel by straining the crystal lattice. It is impossible to make spinel with the normal composition of the natural by the Verneuil process. The reason is that, instead of a boule being made up of one crystal
only. It is composed of a number of individual crystals, each of which has a different orientation; this means that as soon as the boule cools, cracking is a certainty. One of the early investigators into the nature of this material concluded that the excess alumina remained in a cubic form known as gamma corundum; this makes commercial synthetic spinel a mixed crystal of magnesium aluminate and gamma alumina. Since
the composition of the synthetic differs appreciably from that of the natural, there is some question as to the accuracy of the use of the term synthetic for this product.
Synthetic spinel is made in a wide variety of colors; almost all hues, with the exception of purple, are represented. Also, deep tones of red, ranging from ruby colors to garnet colors, are not ordinarily made. On several occasions, ruby-red specimens have been reported, and perhaps will become significant commercially. One of these was apparently produced from solution. The crystal was grown attached to a platinum plate, and had a higher-than-normal S.G., and a very high R.I. (1.75); however, X-ray diffraction patterns were identical with those of natural spinel. The only other stone of this description that has ever appeared was a cut specimen that was identified in Paris. It is not clear whether these two examples were hydrothermal laboratory experiments or whether someone is making stones that pass for natural spinels and is keeping his success of the synthesis a secret.
Later, Dr. Edward Gubelin, of Lucerne, Switzerland, described synthetic ruby-red spinels that were in contrast to any other synthetic spinels encountered in the past. These red spinels have an index and S.G. almost identical with those of natural red spinel; this marks them as distinctly different from the usual Verneuil synthetic which is characterized by a high percentage of aluminum oxide. When received for identification, several small (all less than. 25 carat), calibre-cut red synthetic spinels from Brazil, they were presumably the same type of material observed.
In 1960, it was reported that boules of 40 grams or 200 carats were in existence, and cut stones of 2 carats and 5.5 carats were noted. The refractive indices of this material is in the range of 1.722 to 1.728, with a specific gravity of 3.60. Under magnification for immersion, plain curved striae is seen (Figure 6) along with small spherical bubbles and a few larger whorled or "profiled" or irregularly drawn out bubbles. Also noted, are two-phase tiny flat cavities, containing a bubble and either liquid or gas, often joined by a thin tube. The expected anomalous double refraction is seen when the use of the polariscope is employed. Under both long-wave and short-wave ultraviolet, a crimson color is exhibited as well as a pronounced phosphorescence upon exposure to X-rays. In 1954, opaque blue stones that bore a close resemblance to lapis-lazuli were introduced from Idar-Oberstein, West Germany; these were proved to be essentially synthetic cobalt-colored spinel but made by a new process. Under magnification it is obvious that the product is not a single crystal but an aggregate of many crystal grains. Information published at the time of its introduction called the material "lapis-lazuli-colored sintered synthetic spinel". The data also mentioned that finely ground synthetic spinel is sintered (that is, semi-fused at 2135°C) and then re-compacted by pressure. It is interesting to note that the pyrite inclusions in lapis-lazuli can be imitated by adding gold filings, although most of it is produced without it. In this case, it seems ironic that "fool's gold" is a sign of a genuine stone and gold is a sign of an imitation material. As a gem material, this product has every desirable attribute. The color is pleasing and clear and, with a hardness of 8, its durability is excellent. It is made in flat plates two or three millimeters thick and in spheres of several millimeters. A new imitation lapis-lazuli is being produced and marketed by Pierre Gilson S.A. Lapidatres. This manmade material is partially filling the needs of the trade and taking the place of the sinteres synthetic blue spinel.
Another form of synthetic spinel is made by reheating colorless material long enough for some of the alumina to separate, probably as small corundum crystals. This imparts a cloudiness that gives rise to a realistic adularescent effect, making an excellent moonstone imitation. A preferred orientation of inclusions produces a star effect, which may be heightened by a very thin metallic coating on the flat base of cabochons; usually, it is so thin that it is transparent.
Synthetic spinel continues to be made for several reasons. First, because it is slightly softer than synthetic corundum, it can be fashioned more rapidly and less expensively. Secondly, several of the colors in which it is made are better imitation for a number of natural stones than those made in synthetic corundum.
The difference between the boiling point and the melting point of synthetic spinel is considerably greater than that between synthetic corundum; therefore, the manufacture of spinel does not require as much delicacy of control to prevent the formation of bubbles as is required with corundum. As a result, it is not unusual for bubbles to be entirely absent in spinel. When they do occur, however, usually they are minute and distributed randomly throughout the stone, although not as highly concentrated as in Figure 7. Irregular, somewhat elongated bubbles also may be encountered (see Figure 8), as well as ones that appear angular because of numerous, flat faces (Figure 9).
Also, since under almost all conditions it is free from anything comparable to the curved striae encountered in certain varieties of synthetic corundum, other methods are necessary to identify it conclusively.
One of these methods is the combination of high R.I. (1.727 to 1.729), compared to approximately 1.718 of the natural, and anomalous double refraction in a patchy pattern, which a tester soon recognizes when a stone is viewed in the dark position of the polariscope (see Figure 10).
A combination of these two characteristics may be considered as proof of synthetic origin. Similarly, a 1.715 to 1.720 R.I. and a strain-free reaction in the polariscope is sufficient to identify natural spinel. The natural is comparatively rare without inclusions of any kind, although perhaps less rare in an internally flawless state than most of the other natural stones. In other words, flawlessness is perhaps more common for synthetic spinel than for most other gemstones.
The color filter, which was originally designed to separate glass imitations and doublets from natural emeralds, has been outmoded by the introduction of synthetic emerald and certain plastics that appear red under the filter. Therefore, because of the filter's usefulness with synthetic spinel, there has been some attempt to change the name to "spinel filter", rather than emerald filter. The advantage of the filter with synthetic spinel is not as a means of separating it from natural spinel, but as a simple test for distinguishing several of the more common imitations from gems they imitate. Pale-blue synthetic spinel, which is commonly used as a substitute for aquamarine, appears pale red under the filter, depending on the depth of color of the stone. Aquamarine does not react. However, the deep blue Maxixe-type beryl is a weak red when viewed through the Chelsea (emerald) filter. Dark-blue synthetic spinel, which is often used to imitate both natural and synthetic sapphire, appears dark red under the filter, whereas neither natural nor synthetic blue sapphire shows the red reaction.
There is another method by which synthetic spinel may be identified readily in certain instances. The cobalt coloring in blue stones can be detected by the use of the spectroscope, for the cobalt lines are usually prominently visible at 6350 A.U. in the orange, at 5800 A.U. in the yellow, and at 5400 A.U. in the green.
In dark-blue and lapis-colored material, the bands in the orange and yellow are particularly broad about twice the width of the line in the green. In light-blue material, the three bands are much less strong. Occasionally, a bright-red fluorescent line is visible in the deep red. Natural blue spinel shows a band in the orange at 6320 A.U., with a stronger and sharper band at 4800 A.U. and a strong broad band at 4590 A.U.; this is caused by ferrous iron. Synthetic yellow spinel shows two lines in the blue-violet at 4450 A.U. and a very sharp line at 4220 A.U. in the violet. Greenish-blue material exhibits a spectrum that combines the one in the blue caused by cobalt and the one in the yellow caused by manganese. Alexandrite like stones have a broad absorption in the yellow-green.
The detection of lapis-colored sintered spinel is not difficult. First, the color is very intense and to the uninitiated, seems too reddish. Also, the luster is too high for lapis. With instruments, a quick and positive test is refractive index (approximately 1.725), which is higher than the 1.5 expected of lapis 1.68 expected of lapis rich in diopside. Under the color filter, the stone appears dull red, unlike lapis or dyed chalcedony ("Swiss lapis") which remains unchanged under the filter.
It is not difficult to separate synthetic spinel from most of the gemstones it imitates, for most of them are identifiable by the key property of refractive index. The stone it may resemble most closely in appearance is the one with the closest R.I. of the important gems; alexandrite. The usual so-called synthetic alexandrite is a synthetic sapphire that bears little resemblance to the genuine chrysoberyl variety, in that it changes from a color similar to that amethyst to a gray-blue. The rare synthetic spinel changes from brownish red to green; therefore, they are easily mistaken in appearance. Although the 1.74 to 1.75 R.I. of alexandrite is close to the 1.73 of synthetic spinel, the strong trichroism of the natural identifies it readily.
In identification, the presence of bubbles in a singly-refractive material may cause the tester to conclude quickly that he has glass. Since glass with an index over 1.70 is exceptionally soft and almost useless for gem imitations, it is very rarely encountered; moreover, its high lead content would give it an S.G. near 5.00.