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Synthetic Emerald




The high value of emerald made it an obvious target for early efforts at synthesis. When the Verneuil method for synthesizing corundum was developed, the next obvious step was to powder beryl, add chromium oxide, and attempt to grow boules of emerald. The result was not crystalline material, but glass with a beryl composition; it would not crystallize under these conditions. Later, numerous efforts were made to grow beryl by methods comparable to those used today to produce synthetic quartz. The synthesis of beryl did not seem to offer serious problems to the early investigators, who were content to produce tiny crystals; it was only when attempts were made to produce emeralds of a size and quality suitable for gem use that greater problems were encountered.

History of Synthetic Emerald

From the information gained before World War II about the production of the material called "Igmerald" by the G. I. Farben Trust in Germany, and from reports of successful efforts of earlier scientists, such as Hautefeuille and Perrey in 1888, and Nacken in 1912, to synthesize beryl, it seems valid to conclude that the flux-grown process is used to produce the Chatham, Gilson, and Zerfass products, while the Linde and Lechleitner synthetics were manufactured by hydrothermal methods. In emerald synthesis, the Verneuil flame-fusion technique has not been successfully employed commercially. However, boules of synthetic emerald were reportedly produced by the flame-fusion (Verneuil) technique in 1963 by the Hughes Research Laboratories in California.

Essentially, the process used to produce the first synthetic emerald termed "Igmerald" has been described as a melt-and-diffusion process. A platinum crucible is used and the oxide mixture of beryllium oxide and alumina is placed in it. On top of this is placed the melt, consisting of acid lithium molybdate. Some pieces of silica glass are added to the lithium molybdate to furnish the SiO2; chromium salts are added to provide the color. When the optimum temperature of 800°C (1470°F) is reached, the oxides at the bottom of the bowl are dissolved by the melt and, by diffusion, brought to the surface. The oxides then act with dissolved silica particles and clusters of synthetic emerald are formed.

In order to avoid random crystal growth, seed crystals of synthetic emerald are placed below the surface of the melt and held in position by a platinum sieve. As the growth process continues, additional oxide mixture must be added periodically. This is facilitated by a vertical tube of platinum which allows additional material to be supplied to the bottom of the crucible. As synthesis is completed, the enlarged emeralds are removed from the melt, cleaned, separated and categorized according to size. The length of period required to produce an emerald crystal large enough to be cut and fashioned into a one-carat stone is estimated to be one year.

None destructive infrared spectroscopic tests indicate that no water occurs in flux-grown emeralds and that water is present in the hydrothermally grown synthetics. Therefore, on the basis of laboratory tests, the manufacture of Zerfass, Chatham, and Gilson processes are similar to the "Igmerald" of the I.G. Farben process. In short, it is a flux-grown process, and differs greatly from the hydrothermal, in that the material to be synthesized contains no water and is dissolved at high temperatures in a molten flux that is in a solid state at room temperature. When melted, the beryllium and chromium oxides dissolve in it fairly readily.

Chatham crystals

Chatham claims to start with commercial beryl of an essentially colorless type and to add a certain amount of chromium oxide to the flux from which the crystals are grown. It would seem from this that the use of low-grade beryl accomplishes the purpose of furnishing the feed stock without requiring the long and involved purification and calcination of the powders required by the Verneuil method. The color of the best Chatham stones is an attractive deep green that resembles fairly closely some of the top-quality Muzo emeralds, but they tend to be even more bluish than these natural stones. It has been stated by Chatham that he could alter his product to resemble more closely the Colombian material, if he wished to do so.

It is interesting that Chatham got his start on the synthesis of emerald at the age of fifteen. As many boys do, he became interested in producing explosive concoctions. After one particularly close brush with serious injury, his father forbade any further efforts along these lines. Thus, the young chemist turned to other subjects of interest, chief of which was the manufacture of emeralds. A chronological account of his efforts is illuminating. In 1930, he produced colorless beryl in a maximum size of approximately 1 x 1 millimeter. In 1933, imperfect emerald crystals of about the same size were made. Well formed 1 x 1 millimeter crystals appeared in 1934. The size increased in 1941 to fifteen carats, but the quality remained the same. In 1947, both size and quality improved and semi-commercial production began, with the largest crystal weighting 42 carats.

The Chatham crystals today are characteristically well-developed hexagonal prisms or groups of prisms, frequently with concave faces. A large crystal (about 400 carats.) of recent vintage is shown on the frontispiece of this assignment. Chatham is now producing in commercial quality, and a production in excess of 15,000 carats per month is available, including gem-quality cut stones in excess of six carats and fine quality faceted stones over ten carats. He is also experimenting with a variety of beryls of other colors, less from the consideration of possible commercial production for gem purposes than for use in science.

Gilson

Pierre Gilson of France established his laboratory in the 1930's for purposes of research in mineral chemistry in the field of ceramics. It was not until 1956 that he attempted to make synthetic emeralds. Early attempts by Mr. Gilson were thought to be hydrothermal, but his present day synthesis involves a flux-grown process strangler to those previously mentioned. He has reported producing clean stones up to 23 carats in weight. The crystals are in the form of hexagonal plates grown on basal cut seed plates or long flat plates approximately three inches long, cut parallel to the prism direction. The crystals grown on these seeds are formed either long flattened hexagons, or equidimensional hexagonal plates. The usual size is 350-400 carats, but crystals of over 600 carats have been made.

The, Gilson process begins with a thin slice of synthetic emerald chosen for its clarity and freedom from defects. The crystal is then grown and enlarged in a saturated solution in a molten flux in which the essential oxides of beryllium, aluminum, and silicon are dissolved at atmospheric pressure and temperatures of about 800-1000°C. The beryllium aluminum silicate crystallizes out of the saturated solutions in a spiral growth pattern in a hexagonal arrangement. The rate of growth is estimated to be only one millimeter per month.

Flux emerald

A new flux emerald is now being called Crescen Vert Emerald produced in Japan by Kyoto Cerarnio Inc. Its properties are: refractive indices 1.560 to 1.5133, birefringence 0.003, specific gravity about 2.66, appears red under long-wave ultraviolet and a weak chalky yellow under short-wave, and contains typical flux and/or minute tube like inclusions parallel to the optic axis.

Hydrothermal synthesis

Hydrothermal synthesis has been undertaken successfully by many different scientists at various times for a period of more than one hundred years. Perhaps the man who contributed most to the advancement of this technique, and to whom later workers owe the greatest debt, was an Italian named Spezia. His work led the way to the efforts of the I.G. Farben group in their production of synthetic emerald in the early to mid-1930's, to those of the Bell-Laboratories in synthetic quartz, and presumably, to Gilson, Chatham and Linde's initial efforts as well.

Spezia accomplished his major work only a short time after the Verneuil process was introduced. He placed fragments of quartz in one end of a silver-lined autoclave in a weak sodium-silicate solution and placed a quartz seed "crystal at the upper, slightly cooler end of the container." By applying constant heat over a period of many months to the lower end of the autoclave, the quartz fragments were slowly taken into solution and re-deposited on the seed crystal at the cooler end. Spezia started with a seed crystal about five millimeters long and added twenty millimeters long, or about four-fifths of an inch, to the seed.

The Bell process is essentially the same as that used by Spezia so many years ago. The hydrothermal synthesis employed by "Linde and Lechleitner is much lengthier-and more exacting than the Verneuil process in many respects. The reason that the growth requires a period of months, rather than hours, is that silicates in general are exceedingly insoluble, and are taken into solution only very slowly. It is interesting that a very slight temperature variation makes the difference between deposition on the seed crystal at the cooler end of the autoclave or dissolution at the warmer end. Anyone who has seen the monoclinic crystals of rock candy crystallized out of a sugar and water solution can visualize the principle of hydrothermal growth. The essentials of the process, water and heat are present, even in this rather simple example.

The Linde synthetic crystals are grown in autoclave in an aqueous solution by suspending a seed crystal from thin wire. Although a slow process, some very rich emerald-green crystals result. The time taken for growth, and the need to replenish the solution from time to time, necessitates high cost. It has been reported that in later batches wire was not employed.

About 1960, a competitive product appeared in America; it was prefaceted beryl with an overgrowth of synthetic emerald, grown by the hydrothermal process. Early material was re-polished only on the table; presently, however, most is nearly fully re-polished. The result resembles a pale natural emerald to the unaided eye. Made in Austria by the chemist, Johann Lechleitner, it was brought to the United States under the name '''Emerita". Later, Linde Air Products took over its distribution. Linde first used the name "Linde Synthetic Emerald", but the Federal Trade Commission enjoined them from that term. Prior to Linde's exit from the manufacturing of synthetic gem materials for the jewelry trade, they had described their emerald more fully as a beryls tone with a synthetic emerald overgrowth.

There are currently three different types of synthetic products developed by Mr. Lechleitner- the overgrowth type-natural beryl core and synthetic layers; the sandwich type-- multiple plates of natural and synthetic materials with synthetic growth layers; and a full synthetic without a core, of natural beryl in which visible striation can be seen.

An additional hydrothermally produced synthetic emerald is a stone colored with vanadium rather than the usual chromium. The color is a rich grass green, and its properties coincide within the range of the Linde synthetic. The major problem that arises with this stone is that many gemologists are not in agreement whether
to call this stone emerald or green beryl due to the nature of the coloring substance.

Of additional interest, it should be mentioned that emerald synthesis has been investigated for use in maser, laser and semiconductor research. The laboratories concerned in this field include Bell Telephone Laboratories (Murray Hill, New Jersey), General Telephone Laboratories (Palo Alto, California), and Hughes Research Laboratory (Malibu, California).

The separation of synthetic from natural emerald, it will be recalled, was discussed in detail in Test and Identification of Emerald this included a description of the characteristic inclusions of the artificial product. However, photomicrographs of these diagnostic features, which did not accompany the discussion, are presented more appropriately at this time. To review this portion of the emerald assignment briefly, it was said that the natural stone was characterized by three-phase inclusions and included crystals of calcite, mica, pyrite or actinolite, none of which is ever present in the synthetic.

The CHATHAM product contains numerous solid flux feathers, usually veil like formations which may be likened to waving curtains (three different magnifications are shown in Figures 14, 15 and 16). The wisplike or veil like inclusions are actually solid flux.

Also present, may be a series of parallel bands or zonal lines that are straight or angular in conformity to the hexagonal prism see Figure 17. These bands or lines may sometimes appear as cavities of hexagonal outline in the direction of the main crystal axis.

Included crystals of phenakite (beryllium silicate) or platinum may be found, but can easily be confused with the pyrite inclusions of the natural stone, since platinum occurs in the cubic system (see Figure 18). In appearance, the Chatham synthetic emeralds have a rather distinctive blue-green color, though Colombian emeralds of the same tint have been noted.

The LINDE product has characteristic veil like inclusions and nail like cavities, with the head of the nail formed by a single, or a group of phenakite crystals. If there are several of the nails, they are always parallel (see Figure 19).

The inclusions to be found in the GILSON group of synthetic emeralds, include veil like flux wisps as well as a number of crystals of platinum or phenakite scattered throughout the stone (see Figure 20, 21 and 22). There may also be hexagonal fragments of ilmenite (an iron-black mineral with a metallic luster).

(NOTE: A chart with the physical properties as well as the characteristic fluorescence, for the CHATHAM, LINDE and GILSON products, may be found at the end of this section).

Some of the GILSON emeralds show colored strain patterns when viewed between crossed Polaroids. These patterns do not clearly extinguish every ninety degrees (see Figure 23).

The color of the earliest Gilson's are rather yellowish green, displaying dichroic colors of yellow-green and bluish-green. The table is cut at right angles to the optic axis as in the Chatham product.

The two other types of Gilson emeralds are bluish-green; "one of these groups remains inert under long-wave ultraviolet. This non-fluorescent emerald often contains the wispy veil like inclusions usually found in the other synthetics (see Figure 24), and exhibits a characteristic absorption spectrum with a line at 4270 A.U. (See Figure 25).
As mentioned it is the addition of iron that gives new type its unique properties.

The LECHLEITNER products can be identified by their properties as well as the unique appearance of its component layers.

The core or seed plate used is either natural colorless beryl, pale aquamarine or pale yellow beryl. Upon this core is grown a layer of synthetic material. The crown of the stone is then polished upon removal from the autoclave. The refractive Indices of the overgrowth product are in the range of 1.575 to 1.581 (birefringence of 0.006), with a density of 2.69.

Another Lechleitner product is a "sandwich" arrangement consisting of a central seed plate of synthetic emerald grown on an additional plate of natural or synthetic beryl. Upon these are deposited numerous layers of synthetic material. The refractive indices of this type are 1.566 to 1.570 (birefringence of .004), with specific gravity of 2.68. The overgrowth and "sandwich" types of synthetic show, upon microscopic examination or immersion the discussed planes of separation (Figure 26 and 27). These irregularities are cracks or fissures as shown in Figure 28.

In reality, the "sandwich" and overgrowth types represent differences in the refractive indices of the various components of the stone, causing optical boundaries. Under long-wave ultraviolet, the synthetic emerald will rim the stone with red fluorescent light. When these stones are viewed through crossed Polaroids, a wavy extinction pattern, similar to that exhibited by synthetic spinel, is often seen.

Lechleitner's third type "the full synthetic" has refractive indices of 1.569 to 1.574 (birefringence of .005), with a specific gravity at 2.70.

Reports in the literature indicate that some recent Lechleitner emeralds are overgrowths on colorless topaz. They can be recognized by having both higher refractive indices and specific gravity than other Lechleitner synthetic emeralds. The R.I. is usually about 1.592 to 1.600 and S.G. about 3.40.

Table of Properties of Synthetic Emeralds

Method Name R.I. Birefringence S.G. Long U.V.
Flux Grown Chatham 1.561-1.564 0.003-0.004 2.66 Brick Red
Flux Grown Gilson- Overgrowth 1.564-1.569 0.005 2.65 Orangey Red
Flux Grown Gilson- natural beryl core 1.562-1.567 0.005 2.65 Red
Flux Grown Gilson- sandwich 1.571-1.579 0.008 2.68 Inert
Hydrothermal Linde Hydrothermal 1.566-1.571 0.005 2.67 Strong Red