Corundum is such an important gemstone and so valuable in many of its colors that for centuries many materials have been made to imitate or reproduce it. In addition, many varieties of natural mineral species may be confused with corundum, because of the numerous colors in which it occurs.
The most important problem in identifying corundum is to distinguish the natural from the synthetic, both flame-fusion ruby and sapphire and flux grown ruby.
Flame-Fusion Ruby and Sapphire.
The most important means of separation is provided by characteristic inclusions. They may be two phase (liquid and gas in a pattern that has been likened to a fingerprint), silk like, or crystals or grains, either rounded or angular. Experience will enable the tester to distinguish between rounded grains and gas bubbles by relative relief.
Since the Verneuil process a flame-fusion method, the temperature of the flame through which the powdered alumina pusses to be melted is above the boiling point of corundum. If the flame is too close to the top of the growing boule, the alumina at that point will boil and gas bubbles, usually minute and in groups, will be formed. The difference between the melting point and the boiling point of corundum is only approximately 100°C; therefore, the flame distance must be controlled carefully to prevent excessive boiling. Because they contain gas, the great difference in R.I. between the bubbles and the corundum gives rise to a very high relief between them and the host material; as a result, they stand out as black spots in transmitted light. In dark-field illumination they stand out as bright pinpoints. In most synthetics of recent vintage, the bubbles are either very tiny or absent entirely. Earlier synthetics, some of which are still seen, had large bubbles that appeared doughnut shaped when observed in dark-field illumination (Figure g). Sometimes, gas bubbles have "tails" and rarely are greatly elongated and distorted (Figure h) Rounded, dark grains are sometimes encountered, which are inclusions of alumina powder that failed to melt (Figure i); alone, they resemble some rounded grains seen in natural stones, but usually they are accompanied by numerous tiny gas bubbles and prominent curved striae or curved color banding.
In general, inclusions in both natural and synthetic corundum are most easily seen under 30x, using binocular magnification and dark-field illumination. Thus equipped, a trained man usually is able to resolve bubbles in all but a very rare synthetic. Natural corundum entirely devoid of inclusions or color banding is exceeding rare.
A second difference between the natural and manmade product is in the nature of the growth lines. As the boule grows, its-top retains a curved surface, a condition that seems to give rise to enough difference in homogeneity for the curvature to be visible as an optical effect curved accumulation surface; this is apparent as broadly curved concentric lines in synthetic corundum of recent vintage (Figure j). Both early synthetic made in small boules (Figure k) and stone cut from the area around the boule tip (where the boule was started from a seed) usually show tightly curved striae; frequently, the striae are accompanied by elongated bubbles oriented perpendicularly to the striae (Figure 1). If present, growth lines in the natural stone conform to the hexagonal pattern of corundum. These are not always visible; when they are however, sometimes only one set of parallel lines, corresponding to one prism face, is evident. If the boule from which the synthetic was cut was large and the radius of curvature large (Figure m) there is a possibility of confusing straight color banding in a natural stone with the appearance of striae in the synthetic; fortunately the curvature is usually apparent even in very small stones.
Curved striae are not present in all varieties of synthetic corundum for example, although curved color banding usually is seen in the blue material, the striae effect is seldom noted. Curved striae are particularly prominent in synthetic alexandrite like sapphire and ruby, but in the other colors they are seldom so apparent. However, evidence of curved color banding has been reported in some of the synthetic yellow material. Colorless, green and very pale stones of other colors never show curved striae or color banding of any kind by any present method of examination. Thus, one effect, or the other is visible in the strongly dichroic stones, but not in the weakly or non-dichroic Varieties.
When examining a stone for curved striae or color banding, it is essential that it be observed in a manner that permits the interior to be viewed; otherwise, it is rather easy to confuse these characteristics with polishing marks. If a set of straight color bands meets another set at the usual 60°angle of the natural, hexagonal stone, it is proof of natural origin. One cause of possible confusion is the fact that even the synthetic is subject to gliding, or twinning; in other words, sometimes a movement or an apparent movement of rows of atoms with respect to others produces a visible plane that appears as a straight line when viewed end on. This may occur in the synthetic as well as in the natural, but it is much more common in the natural.
Striae, color banding and other growth lines are most easily viewed under magnification; if they are difficult to locate, immersion in a high-index liquid usually will make them more apparent. Particularly advantageous for this purpose is methylene iodide, which has a refractive index close to that of corundum. When attempting to locate growth phenomena with the dark-field microscope, either with immersion or in air, it sometimes helps to place a cleaning tissue or similar material over the light and under the stone or receptacle; this will provide a diffused light-field source of illumination.
Blue sapphires often may be distinguished from synthetics of the same color by the use of the spectroscope. In the natural stone, which contains iron, there may be from one to three lines in the blue, centered at approximately 4500, 4600 and 4700 A.U.; the 4500 line is the most persistent, being present even in some pale stones. In blue Australian sapphires, all three lines show clearly, becoming bands of considerable strength, whereas in blue Ceylon stones often only the 4500 band is visible as a fine line. These same three bands also are strongly evident in green sapphire. In addition, they are sometimes visible in yellow Australian sapphire but they may be very weak, whereas they are nonexistent in yellow Ceylon sapphire. No distinguishable difference in absorption spectra have been noted for other colors of natural or synthetic corundum.
Under long-wave ultraviolet, it has been found that natural yellow sapphires from Ceylon fluoresce a distinct yellowish orange color. Synthetic yellow sapphires are either inert or show brick red under long-wave, and orangey-red under shortwave ultraviolet. Under short-wave ultraviolet, flux-grown rubies fluoresce closely or the same bright red of the natural stone; hydrothermal rubies fluoresce an intense red as compared to the natural stones; the flame fusion ruby fluoresces an even slightly more intense red. It must be mentioned then, that concerning red synthetic corundum, caution should be exercised when using fluorescence as a test. Under shortwave ultraviolet, the larger majority of synthetic blue sapphires fluoresce a dusty or chalky yellowish-green to a powder-blue color when viewed in a dark room against a dull black background such as black velvet. Very seldom do natural blue sapphires fluoresce, but if present, the color usually is very weak. When natural sapphires fluoresce, usually there is no iron absorption line at 4500 A.U.
A test that has been used to some extent to detect flame fusion synthetic rubies is phosphorescence under X-rays. Both synthetic and natural ruby fluoresce strongly when exposed to X-rays, but the artificial product continues to glow after the radiation is turned off. The use of this test, of course, is limited to a completely equipped gem-testing laboratory.
Synthetic ruby made by the Verneuil process is more transparent to short-wave ultraviolet radiation than the natural stone. Ultraviolet is referred to as short-wave when the longer mercury line, at 3600 A.U., is filtered out and the shorter, major emission from a mercury tube (2537 A. U.) is used. Colorless and light-pink corundum are reasonably transparent to short-wave ultraviolet, but other colors are opaque. The synthetic, on the other hand, tends to be transparent to this wavelength in all of its colors. This property may be tested in one of several ways. One method is to place the stone table down on a piece of photographic paper in the bottom of a flat pan or dish and cover it with water. The stone is exposed to short-wave ultraviolet for two or three seconds, and then the paper is developed. Of course, the entire procedure must be carried out in a dark room in which the only illumination is a safety light. When the paper is developed, the synthetic will be distinguishable from the natural because it will have passed much of the light, but the natural will be apparent as a virtually white spot. It is best to use known stones for comparison purposes. A similar method is to cut a small hole in an opaque baffle, place the stone over the hole and place directly beneath the hole a material such as scheelite that is fluorescent only to radiation below 3000 A.U. If the gemstone against which the ultraviolet lamp is directed is transparent to ultraviolet radiation, the scheelite will fluoresce; if it is opaque, no reaction will take place. This makes an effective test for synthetic rubies and sapphires, other than pink and colorless.
A rarely used test is to examine the stone parallel to the C-axis (i.e., the direction in which its interference figure is visible) in the polariscope dark position under fairly high magnification. In the dark position, anomalous double refraction is seen in a pattern conforming to the hexagonal structure of the synthetic. This test is sometimes needed when attempting to distinguish between very pale natural corundum and the synthetic (Figure n). As mentioned previously, curved striae are seldom evident in the very pale colors of the artificial product; if such a stone is devoid of bubbles, this method may be necessary. However, the fact that most stones are cut with the optic axis nearly parallel to the girdle makes it is used only as a final resort in gem-testing laboratories.
Because of the lack of transparency of most synthetic star rubies and sapphires these stones may be somewhat more difficult to distinguish from their natural counterparts than the transparent forms, if the "bulls eye" mentioned earlier is missing. With care, however, one should be able to note the curved striae or color bands without too much difficulty. In addition, if the stone is lighted property, bubbles usually will be evident in the synthetic. In natural star corundum, angular inclusions and either zoning or color banding usually is very noticeable. The inclusions that cause the star in the synthetic are rutile needles, but they are so minute that they are visible individually only under magnification in excess of 100X (Figure 0). The silk in natural star stones usually is significantly coarser, and individual needles may be resolved under much lower magnification. Many of the more opaque synthetic star sapphires and rubies display a peculiar cloudy appearance on the surface when observed under at least 30x in reflected overhead light. The effect seems to be caused by irregularly deposited rutile needles, some of which are concentrated into definite cloudy patches.
A particularly troublesome synthetic ruby or sapphire is one that been "quench crackled" (Figure p). This is done by heating a polished stone and plunging it in to cold water or a saturated solution, from which the chemical crystallizes in the cracks formed by quenching, giving arborescent forms to the deposit. Sometimes, the heated stone is immersed in a dye, which imparts color. A stone treated in this manner appear to have natural inclusions and fractures and thus be misidentified, particularly if it is cut in one of the styles used for natural stones. The practice has been used for a number of years to falsify solitaire size stones, but more recently it has been used for imported calibre-cut stones. Usually, the appearance of the cracks in larger stones is recognized by experience. Magnification generally discloses curved striae or growth lines or bubbles, as in ordinary synthetics. The importer of calibre sizes has a more difficult task in separating crackled stories from large lots of natural rubies or sapphires. Especially troublesome stones, both large and small, should be immersed in methylene iodide, to minimize the effect of the cracks while searching for identifying inclusions. Another suggested test is to use the shortwave ultraviolet unit and note the greater fluorescence of the synthetic when observed in a dark room.
Several frequently encountered characteristics of natural and synthetic stones can be regarded as indications of origin but not as Proof. For example, the synthetic is usually cut so that the optic axis is approximately parallel to the table of the stone; in the natural, on the other hand, it is almost / always perpendicular to the table. In the synthetic, the optic axis is parallel to the split halves; therefore, size plus convenience calls for the table to be parallel to the optic axis. Thus, if interference colors are noted through the table when examining a stone other than a star in the polariscope, it is more likely to be natural than manmade. For the same reason, dichroism usually is visible through the table in the synthetic but not in the natural. At best, this is an indication of origin.
The synthetic stone often is polished so rapidly that the heat generated causes irregular cracking, usually on or near facet junctions; frequently, however, they may be present over the entire facet (Figure q). Such cracks are seldom encountered in natural stones. Synthetics frequently are cut either in the brilliant style or, even more frequently, in regular millimeter size (e.g. 8 x 10 mm, 10 x 2 mm, or 12 x 14 mm.) in the oval brilliant, step cut, scissors cut, etc. Natural stones are fashioned more often in an oriental, unsymmetrical cut or in a symmetrical emerald cut; less commonly, they are cut in the round brilliant style. Orientation, of course, is of no value in distinguishing between a synthetic and a natural star stone, because both must be oriented in the same direction in order for the star to be evident.
These indications ere listed not as guides for identification, but merely to provide some assistance in putting the jeweler or guard against misrepresentation. Indications suggesting synthetics call for immediate testing, if the stones are offered as natural.
Since a hydrothermal process for manufacturing synthetic ruby crystals introduces solid as well as liquid and gas inclusions during growth, the internal features may resemble those of natural stones. However, these are not to be confused with the three phase inclusions seen in natural emeralds. In view of this fact, considerable experience is required before relying on magnification of these stones.
The most easily observed distinguishing characteristic of hydrothermal synthetic rubies is their fluorescence under short-wave ultraviolet radiation. In this regard, they are the same as flame-fusion synthetics, which glow much more strongly than natural rubies of the same color. When conducting this test, it is advisable to use a small natural ruby and a similarly colored synthetic ruby as control stores. As mentioned before, the stones should be placed on a dull black surface in a dark room. An occasional difficult hydrothermally produced ruby may require additional laboratory tests, including X-ray phosphorescence and transparency to the 2537 A.U. ultraviolet. The reactions in both of these tests are similar to those described previously for flame-fusion synthetic rubies.
An additional characteristic of the hydrothermal types of internal features is often a trace of a "seed" used in the manufacturing process, and the stones will show rather typical feathers somewhat reminiscent of those seen in synthetic emerald.
In flux-grown ruby, there is seen a typical design of a finger-print-pattern inclusion (Figure r) and these sometimes appear like droplets or dashed lines (Figure s).
Some of the flux-grown types incorporate a large natural seed in the manufacturing process and when this is the case, deceiving natural silk of coarse needle-like inclusions may be present. However, high power microscopic examination of the material surrounding the seed will prove to exhibit the usual veil like inclusions.
This newer type of synthetic often shows a greater transparency to short-wave ultraviolet light than their natural counterparts. Testing may be carried on in a darkened room, placing the stones on a sheet of photographic paper which is immersed in a flat-bottomed dish of water to depth sufficient to cover the rubies. The stones are then exposed to light from a short-wave ultraviolet unit. Subsequent images of synthetic rubies on developing paper will appear dark with a marginal white rim; natural rubies will appear completely white. In reiteration, it must be mentioned, just as with the other synthetic rubies, the flux-grown stones have the density and refractive indices similar to those of natural stones.