Test and Identification of Emeralds

Unquestionably, the materials that are most likely to be confused with emerald today are the synthetic emeralds of Chatham, and Linde, both of the United States, and the Gilson synthetic emerald produced in France. More than one test is usually required to effect a positive separation, but these tests generally are not particularly difficult to apply nor to interpret. Lower refractive indices, birefringence, and specific gravity in most synthetics, plus wispy inclusions, and fluorescence are the key tools of separation. Recent additional tests that prove helpful are the absorption spectra and transparency to X-rays.

The refractive index range of natural emerald may be as low as 1.565-1.570 (rare), to a high of 1.590-1.599 for the Sandawana material; however the usual R.I will be in the area of 1.577-1.583. Note that the birefringence can be as low as 0.005 to a high of 0.009, with the most common figure being 0.006. The indices of synthetic emerald vary, and special attention should be given to these readings. The Chatham synthetic emeralds give readings in the range of 1.561-1.565, whereas the Gilson may be as low, but often in the 1.564-1.569 area: both have a birefringence of 0.003 to 0.005. Some Gilson emeralds however, give readings as high as 1.571-1.579 with a birefringence of 0.008. The Linde Hydro thermals give readings as low as 1.566-1.571, and as high as 1.572-1.578, with a birefringence of 0.005 to 0.006. Most today are near the higher end in indices and birefringence.

The specific gravity of natural emerald ranges from a low of 2.67 to a high of 2.75, with a normal of 2.71. A specific gravity liquid set at 2.67 is often helpful in separating most natural emeralds from the Chatham synthetics and a few others. The specific gravity of the Chatham synthetic emerald is usually constant at 2.66. Most Gilson synthetic emeralds have a specific gravity in the range of 2.65-2.66, but some may have a density as high as 2.68-2.69. The Linde Hydrothermal has a density of 2.67-2.69.

Prior to recent discoveries, a helpful test in the separation of natural and synthetic emeralds was fluorescence under long-wave ultraviolet light. Natural emeralds are most often inert, or may exhibit a weak orangey-red to violetish-red. However, there is now a group of Gilson synthetics that are also inert, and as all the other types of synthetic emeralds are known to exhibit characteristic fluorescence, care must be used in application of this test. Under long-wave ultraviolet light, the Chatham's fluoresce brick red, certain groups of Gilson's vary from a strong orangey-red to red, and the Linde Hydrothermals a strong red.

Another test that is no longer reliable for separating natural from synthetic emeralds is the transparency to short-wave ultraviolet, since in both, opaque and transparent reactions are encountered. Also, when testing for transparency to X-rays, both naturals and most synthetics are transparent with the exception of the group of non fluorescent Gilson's. When tested with X-ray, these stones are opaque. The opaqueness of this group of Gilson's is apparently due to the addition of Iron which also accounts for its lack of fluorescence, and its increased refractive index and specific gravity. The presence of iron also gives this stone a characteristic absorption spectrum with a line at 4270 A.U., and makes testing with the spectroscope very important in its identification.

The inclusions in flux melt synthetic emerald are sometimes confusing natural appearing at first glance. These inclusions are made up of solid flux, usually distributed in a veil-like or wisplike pattern. Phenakite crystals and platinum crystals are often present. Zoning is quite common and these zonal lines may be straight or angular in conformity to the hexagonal prism. The hydrothermal material may also contain clear crystals of the beryllium mineral phenakite, often with long, thin, cone shaped spaces extending from them. The Chatham synthetic often contains crystals of platinum, with a white, cubic appearance. The source of the platinum is thought to be the makers vessel. In contrast, natural emeralds often show three-phase inclusions irregular spaces or cavities containing a liquid, a gas bubble, and a tiny, flat platelike crystal. (In Colombian emeralds the crystal in the three-phase
inclusion appears square or rectangular in shape, but must be resolved under at least twenty magnification.) Natural stones may also contain calcite, actinolite, or mica plates or slender tremolite crystals. Nothing closely resembling these inclusions are found in the synthetic. Metallic inclusions of pyrite with a brass-yellow to golden-yellow cubic appearance are also another possibility in the natural stone.

In 1961, an additional synthetic appeared in America that had been known as the Lechleitner product. For a short time before the development of the Linde synthetic emerald, this was handled by the Linde Company. It is a perfected beryl with an overgrowth of synthetic emerald. The resulting stone bears a resemblance to a pale natural emerald. The synthetic layer has properties more nearly similar to those of the natural than the Chatham synthetic, but it is detected readily by the presence of long, thin, roughly parallel cracks that extend from the surface through the overgrowth layer. When immersed in liquid, the outer layer of synthetic emerald is usually clearly visible. Under long-wave ultraviolet, the synthetic emerald coating will fluoresce a weak orange to red, the intensity depending upon the thickness of the coating. The most intense fluorescence is usually seen in the girdle area.

Of interest to the manufacturing jeweler and of academic interest to the gem-tester, is the report by Chatham that the synthetic emerald is supposed to withstand heating to a red heat without damage, whereas the natural usually shatters under this treatment. Tests made by the GIA, however, proved differently. A natural and a synthetic crystal of equal size were placed on a charcoal block and heated to the same temperature. Both cracked internally after cooling from a red heat but neither shattered, although the natural stone became quite cloudy, and the synthetic resumed its original green color.

Synthetic emerald is so expensive, compared with other synthetics (and, as a matter of fact, with many natural stones), that there is still a demand for other kinds of substitutes that are seldom, if ever, made today as substitutes for other gemstones . For example, triplets (the so-called Soude emeralds) consisting of two pieces of beryl, quartz or synthetic corundum or spinel. Excellent imitations of emerald were also made at one time with garnet-and-glass doublets. A triplet is rather easily detected merely by immersing it in water and examining it parallel to the girdle. In this manner, the colorless or nearly colorless crown and pavilion and the green cement become obvious. (Caution: Immersion in bromoform or methylene iodide may destroy the green color of the cement) Usually, the nature of the substitute is also visible under magnification. If a beryl top and bottom are used of course, the stone will have the same R.I. as a natural emerald; therefore, immersion would be necessary to detect it. The true nature of the piece is particularly likely to be overlooked if it is mounted. Most assembled imitations of emerald have been found to be quartz and quartz triplets. Fused beryl-and-glass doublets have also been offered for sale recently.

Light-colored beryls are frequently set in gypsy-type mountings, either in rings or other forms of jewelry. Such stones may be made to appear much better by the presence of a green foil or a green cement on the pavilion. It is usually possible to detect this added color in a mounted stone in one of two ways. If examination is made in a manner in which enough light is directed at one side of the stone, so that it is possible to use the dichroscope to analyze the light emerging from the stone, the characteristically strong dichroism of emerald will be missing. Weak dichroism mayor may not be evident, however, if the stone has a light color and the color that is seen is the result of the foil backing. By illuminating the stone in the same manner and examining the emerging light with a spectroscope, the absorption bands that one would expect in emerald because of the chromic-oxide coloring agent (especially those bands in the red) would identify the stone as an emerald and the color as genuine, rather than induced by foil.

Unfortunately, emeralds are often "oiled" on the excuse given by some dealers that it is necessary to "restore the natural oils lost in cutting" This is possible only when cracks extend to the surface. Usually, the stone paper or the cloth liner in the paper in which unmounted stones are carried is stained by the oil. An examination of the cracks under magnification usually reveals the coloring agent. It may be bolted out usually, but liquid plastic may be used for this purpose to prevent the boiling from removing the color. A more reliable test of oiled stones involves the use of long-wave ultraviolet light on the suspected stone. If oiled, the treated cracks will fluoresce a pale yellowish.

One of the tests frequently applied to emerald is the use of the emerald filter. This filter is designed so that light is largely absorbed, except in a portion of the red and yellow-green part of the spectrum. Since the emerald absorbs that portion of the yellow-green, the light that passes through an emerald will appear to be red through the filter, in contrast to most emerald imitations. There is, however, no significant difference, in the synthetic and most natural emeralds. Most other green stones, on the other hand, will appear green through the filter. Natural Indian and Transvaal emeralds will appear green also. On occasion, emeralds are encountered that give a satisfactory red reaction but that on close inspection under magnification are seen to be coated. The coating is a colored plastic that gives the same color-filter reaction as that of emerald. Not only are bubbles usually evident in this surface layer, but its softness permits it to be scratched or removed easily. Sometimes, however, a dealer will be deceived by one of these beautifully colored stones and buy them at a price consistent with the depth of color. It is only later that the coating becomes evident and the dealer realizes that he has been cheated. A number of quartz triplets have been encountered in which the green cement layer shows red under the color filter, thus further minimizing the effectiveness of this instrument with emerald.

Other natural stones that resemble emerald in appearance can be separated readily from it by physical properties. These include tourmaline, peridot, demantoid and green zircon. Each of these stones sinks in a specific-gravity liquid of 2.89 (bromoform), whereas emerald floats; furthermore, each has an R.I. range considerably above that of emerald, with zircon and demantoid being above the scale of the refractometer. Tourmaline's R.I. is approximately .04 to .06 higher than most emerald, and peridot is about .05 to .11 higher; in addition, both have much greater birefringence. None of these stones has a really convincing emerald color.

Excellent glass imitations of emerald are made, some of which contain inclusions that resemble those of the natural. Glass with numerous bubbles is occasionally treated in a manner just before hardening that tends to elongate the bubbles and give them a needle like appearance to the casual observer. Sometimes, a material is added that does not melt the same temperature as glass and simulates the appearance of the inclusions in emerald. The R.I. and S.G. of emerald may be duplicated in glass. In this event, it is necessary to resort to the dichroscope or polariscope, for glass is singly refractive and emerald is doubly refractive.

Of the other beryls, a number may be confused with other natural stones of similar appearance and/or properties. Although the physical properties of blue topaz are considerably different, its close resemblance to aquamarine often causes confusion in identification. The R.I. of topaz is usually about two points above 1.69, whereas that of aquamarine is .02 below. A careless reading may therefore lead to an error in identification. This kind of testing mistake is not made frequently; nevertheless, carelessness is an ever-present specter to the tester. Synthetic spinel and tourmaline may resemble either morganite or aquamarine, and zircon looks similar to aquamarine. All of these stones can be separated readily by a refractive-index reading or examination under magnification. The birefringence of tourmaline and zircon is much too strong to be confused with beryl. Synthetic spinel not only often contains bubbles, but it is obviously singly refractive; moreover, blue synthetic spinel turns red under the color filter.

One of the common difficulties encountered by novice testers is distinguishing between yellow beryl and yellow quartz; this can result from a carelessly taken R.I. reading. Since the optic sign of quartz is positive, it gives the usual reading of 1.544; beryl, being negative in sign, gives the usual reading of a high index, or, for yellow beryl, usually about 1.57 or 1.575. Thus, there is a distinct difference in index between the two when the reading is taken in white light under ordinary conditions. Moreover, although the S.G. of the two is similar, beryl will sink in the emerald liquid and quartz will float.

Two unusual gemstones with which beryl may be confused are yellow, transparent labradorite and pink or other colors of scapolite, Labradorite has indices of approximately 1.555 to 1.565 and an S.G. of about 2.70. Careful testing will show that it is biaxial, whereas beryl is uniaxial. Labradorite possesses two directions of easy cleavage, which may be apparent under magnification. Also, it is slightly lower in index and both indices vary, in contrast to the constant high index and variable low index of beryl. Scapolite, which is especially likely to be confused with morganite, has indices of 1.55 to approximately 1.57. The fact that one of the readings is in the 1.57 region could lead one taking a reading in only one position to believe that the stone is beryl. However, the low (.006) birefringence of beryl contrasts with the figure for scapolite, which is near .020. Thus, doubling of back facet junctions in scapolite is fairly strong.


In order to merit the top-quality designation, an emerald must be an intense, medium-dark tone of slightly yellowish or bluish green and have a velvety ("soft") body appearance and a minimum of flaws. Stones of this superfine quality are exceedingly rare; in fact, much less than one percent of the material found can be classed in this grade. In Size of two and three carats and larger, they may exceed the price of diamonds of the same weight and quality. Since emeralds have no fire and less brilliancy than most gems, beauty of color and its distribution are prime factors in determining value. Stones in which lighter and darker portions are arranged irregularly or in layers are much less desirable than those that are evenly colored.

Also, too heavily flawed or "mossy" gems are not desirable. Usually (and ironically), the better the color the less perfect the stone. Light to medium-light tones are much less valuable, even when slightly flawed or nearly flawless; these qualities in size up to five carats are not difficult to obtain. The fact that the finest vivid green and velvety body appearance of emerald occurs in no other transparent stone contributes greatly to its value. Among the important natural gems, jade alone approaches this color, and glass and synthetic emerald are the only manmade materials that imitate it closely.

Unlike emerald, clear aquamarine crystals of great size are often found; therefore, large, flaw less stones are comparatively easy to obtain. Fine greenish-blue gems are the most characteristic of top-quality aquamarines, although the so-called Madagascar blue is considered by many dealers as the most valuable. The price of fine medium and medium-dark stones has increased materially in recent years. These desired tones in flaw less quality are quite rare. The lighter tones, however, can be obtained easily and in almost any size range.

The darker specimens of morganite and golden beryl are scarce and sell quickly to collectors. Colorless beryls are plentiful but have little value.


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