THE SYNTHESIS OF RUTILE AND EMERALD A. E. ALEXANDER Gem Trade Laboratory, Inc., New York
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synthesis of ruby and sapphire was described in THISJOURNAL, September, 1946 (1). Since that date, one wholly new synthetic gem-stone and an improvement of still another have made their appearance in the jewelry trade. These stones are mtile and emerald. RUTILE
Rntile is manufactured by a Verneuil-type furnace of the kind used for processing ruby and sapphire. Chemically, rutile is titanium dioxide, Ti02. The mineral as it occurs in nature crvstallizes in the tetragonal system and is conspicuous for its high density. The gravity falls between 4.20 and 4.26. Most remarkable of all its physical properties are the very high indices of refraction, 2.61 to 2.90. Diamond, by comparison, as a single refracting substance has an index of 2.41. Rutile, like diamond and zircon, has tremendous dispersion. When faceted it reveals a characteristic "life" or "fire," not noticeable in other stones, either genuine or synthetic. Hardness, so important to a gem-stone, is the one physical property in which rutile does not excel. Comparable in this respect to quartz, the hardness approximates 7 on Mohs' scale. Although genuine rutile is usually dark brownish red in color it is also found in nature as a dark blue, violet, or deep green mineral. Genuine colorless rutile is known but is extremely rare. Most natural rutile occurs as a constituent of sand or gravel deposits, usually as one of a, group of heavy minerals, of which garnet and zircon are the best known associates (3). Many tons of this titanium dioxide are obtained from the beaches of Brazil, Madagascar, and Australia and later processed into commercial TiOz. After refining, the oxide finds wide use in the paint and paper industries, and as a filler in face powder. When TiOz is processed int,o titanates, of which there are a Rutile
Diamond
Photo. geimission 01 Tiffany B Co
Photomaph of Synthetic R u t h and Diamond (One C u s t weightE.C~I.
Photo. permlsrlon 01 Carller'ii
A Superb 14-Cuat FlmwluuDen"in. A Podigroed Jewel of Hirtorisll Impo*tans.
Emorold.
great many, the material is utilized by radio and electrical manufacturers for its valuable electrical properties. Due to lack of transparency and sufficient size very few crystals of natural mtile are suitable for fashioning into gem-stones. The synthetic counterpart of natural mtile was first manufactured in 1947. Independent of each other, t v o American industrial firms, the Linde Air Products Company and the National Lead Company, were the first to announce development of synthetic rutile. For Linde, the production of Ti02 was in the nature of a new departure; National Lead, on the other hand, has long specialized in the manufacture of titanium products having wide industrial application. Because of the unusual "fire" that characterizes synthetic mtile the National Lead Company has given their product the commercial name of "Night Stone." This designation is appropriate since synthetic rutile shows to best advantage in artificial light. To date, an absolute colorless stone has not been manufactured, the nearest approach being mtile of a very pale y e l l o ~color. Shades of blue, brown, and red can be produced, and only recently black synthetic rw tile has been developed. The type of equipment used in making prcscnt-day synthetic stones other than emerald was previously described (1). Like the synthetic corundums, synthetic rutile is made in the form of a bode, but in this instance an oxyhydrogen source of fuel cannot be employed, as it is necessary to create a, specific atmosphere for rutile growth. The exact procedure used to manufacture rutile has not been publicly divulged, but it is known that an oxygen-free atmosphere must exist in the Verneuil-type furnace. Since the melting point of rutile is 3315'F., a tem-
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MAY, 1949
255
perature ahove this figure must of necessity he used to produce a viscous mass capable of flow. A 5Gcarat, light yellow rutile houle was donated to the Gem Trade Lahoratoly by Mr. A. K. Seemann, manager of the Crystal Division of the Linde Air Products Company. From this particular specimen it was found that the material could just be scratched with a piece of quartz, indicating a hardness approximating 7. However, for accurately measuring the property of hardness a Knoop tester should be used, which will bring out the true relation between diamond and corundum, for example, having a Mohs' hardness of 10 and 9, respectively. The following table gives the hardness of several well-known substances, including synthetic rutile:
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photo, Permission Synth-tic Boules of Squ-e-Shaped
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Pink Spinel. Pale Yellow Rutils. Woight, Cent*?
mnd Rsd Corundum. Rutil* of 56 &at
Stone Diamond Boron carbide Ruby and sapphire Spinel Precious topaz Synthetic rutile Quartz Fddsp~r Apatite Calcite Gypsum
Mohs' hardness 10 g'/~ 9 8 8
7 7 6 5 3 2
Knoop factor 6000-6500 2,200 1670-2000 1175-1380 1250 900 710 560 360 135 32
Since the Knoop hardness facton for diamond, corundum, and spinel are variable, it is to he expected that synthetic rutile will likewise reveal a Knoop hardness difference. For example, change in chemical composition, as well as in conditions of furnace operation, can result in certain types of synthetic rutile having a hardness as low as FL/2. Should this new gem-stone become popular the question will at once arise as to how this stone can be distinguished from other gems of similar color and brilliauce. First, there is the remarkable brilliance of the stone. For a stone possessing a brilliance so marked can be only one of two kinds-diamond or rutile. The refractive indices of a gem-stone are usually determined by means of a refractometer. Most jewelers' refractomet e n (modified Pulfrich type) read only to 1.80. It is therefore obvious that hoth rutile and diamond will give negative readings on the instrument since both stones have an index far ahove the refractometer's range. Such a test nil1 at least indicate that the stone being tested is not corundum, spinel, or precious topazgems which are nearly or entirely colorle~sand are more often confused with diamond than is generally realized. Diamond is isotropic and therefore single refracting. Rutile is anisotropic and double refracting. Consequently, if hoth diamond and rutile are viewed through the table or crown facets, a marked doubling up of the rear or back facets of rutile will he immediately detected. Diamond, of course, will reveal no such phenomenon. This observation can he made with an ordinary 10-power handmagnifier. A polariscope will also help to distinguish a diamond from synthetic rutile. Diamond remains dark on TO-
tation, regardless of orientation of the stone, while rutile will he alternately light and dark as the material is turned every 90 degrees unless the stone is cut normal to themajor or "c" crystallographic axis. If the stone is loose a gravity determination will aid in distinguishing. It should be pointed out that a layman could confuse the semi-precious gem, zircon, with synthetic rutile. A comparison of the densities of diamoud, rutile, and zircon is as follows: Ziroon.. ................................... 4.68-4.70 Rutile.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.20-4.26 Diamond.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.51-3.53 The specific gravity of the 56lcarat Linde rutile bode was found to be 4.22. X-ray fluorescence is useful iu distinguishing diamond from mtile. All diamonds have a blight to brilliant fluorescence when irradiated by X-rays. Synthetic rutile, on the other hand, is inert when tested under identical conditions. However, it should be pointed out that zircon also produces a brilliant fluorescence when irradiated with X-rays (17,18). X-ray radiography can also he employed as a means of differentiating diamonds from vtile and zircon. Diamonds are very transparent to X-rays. Rutile and zircon are opaque, a fact readily revealed when these stone are radiographed together. When diamond and rutile are photographed side by side diamond shows up to better advantage as can be seen in the illustration. This is remarkable since rutile with its higher indices of refraction and greater dispersion should theoretically he a more brilliant stone. Mention should also be made of the long slender microscopic needles of rutile that are characteristic inclusions in some of our most important genuine gem-stones. Most rubies, sapphires, and many garnets, as well as quartz, contain rutile. Interlocking titania needles, similar in appearance to those observed in genuine corundum stones are an important constituent of the new Linde synthetic star stones ( I , $ , 4,6).
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EMERALD
The only gem-stone which has been successfully crystallized is emerald. Today, perfect hexagonal c r y -
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JOURNAL OF CHEMICAL EDUCATION
tals of this beryllium aluminum silicate, BesAlz(Si0& are produced which can be distinguished from the geuuiue only by very careful examination and by the use of modern scientific methods. The famous French chemist, Hautefeuille, was the first to synthesize emerald. His experiments date hack to 1888 (6). Not much more was done until 1930 when the I. G. Farbenindustrie succeeded in growing crystalline emerald (7). The Germans gave the name "Igmerald" to this product. The crystals, though small, were structurally perfect beryl-the parent mineral of true emerald. German scientists developed good color in "Igmerald," although it was somewhat darker than that accepted by the jewelry trade. However, there was no commercial development and there the matter rested until 1935, when the American chemist, Carroll Chatham, of San Francisco, announced that he had crystallized emerald in his laboratory. Since then, Chatham has sought to improve the color of his lahorato~yemeralds, to increase their size and make them more nearly flawless. Faceted synthetic emeralds a half inch across have appeared on the market and crystalline groups of Chatham emeralds an inch across have been brought to the Gem Trade Laboratory for test. Still larger and h e r specimens are available, according to information received from the authorized dealer in these stones. Although several articles have been written on the Chatham emerald the details of the process by which they are grown have not been disclosed (8, 9, 10). Since I. G. Farben and Chatham emeralds are grown ae actual r~ystals-t,hereby differing from synt.hetic ruby,
Photo. ~ermissicmof Roland Harvey, PIC Magazine Synthetic Emerald Crystals (Lower): Gsnvina Em-rdd Crysteh Not. Similarity. Dark.* Color of Synthetics Is Du. to Inc m d Chromium Oxid. content. (Above).
sapphire, spinel, and 'rutile which are manufactured in the form of a b o u l e t h e process has to be one of growing the beryllium aluminum silicate from a solution. Several articles have been published on the methods used to grow synthetic quartz crystals and it is generally believed that a modification of this technique is the one employed in making synthetic emeralds (11,13). Wooster and Wooster succeeded in synthesizing quartz by suspending a natural quartz seed crystal from a silver wire in a heated and pressurized sodium metasilicate solution to which had been added a mineralizer, as well as a piece of fused silica. After a period of hours a gradual transfer of silica from the particle of fused silica to the quartz seed crystal took place. By this method the synthesis of quartz had been achieved (15). The Bell Telephone Laboratories have also reported success in synthesizing quartz (16). Since it takes a considerable period of time to grow these crystals, a price of $90 a carat is asked for really h e quality synthetic emerald (a bode of synthetic corundum can be produced in a matte1 of four or five hours). This is quite an increase over the price of synthetic ruby, sapphire, or spinel which can be marketed today for three and four cents a carat! If the demand increases for synthetic emerald and if manufacturing methods are perfected a reduction in price per carat can be anticipated. Before discussing ways by which synthetic emerald can he distinguished from the genuine, the basic physical properties of natural emerald should be reviewed. Genuine emerald, the green variety of beryl, clystallizes in the hexagonal system, often as well-defined crystals. This beryllium aluminum silicate has a hardness of 7l/*. The density for high-grade emerald is 2.71. Indices of refraction of Muzo, Colombia, emeraldswhich are the finest quality stones of this species found in n a t u r e a r e 1.570 to 1.580. There is also on the market a lighter-colored Colombian emerald known as Chivor. These gems are usually fairly clear and are characterized by parallel growth hands of light and darker colored green. Indices of refraction for the Chivor gems fall in the 1.569-1.577 range. The density, 2.69, is less than that of Muzo emeralds. A comparison of the physical properties of genuine and synthetic emerald will show that differentiation is possible, although more difficult than it is to tell genuine from synthetic corundum. (1) Color. The green color in the synthetic emeralds examined vas not a true replica of genuine emerald green. Anyone specializing in the sale of genuine emeralds would note this readily and to him the synthetic would be "off color." This ability to recognize the real from the spurious on the basis of color alone is only gained by years of experience in handling many thousands of carats of the real gem. In the last three years an improvement in the color of synthetic emerald has been noted, but it is not yet an exact duplicate of the natural emerald green. (2) Inclusions. Genuine emeralds invariably contain
MAY, 1949
microscopic voids, usually quite irregular in outline. These inclusions, which may occur singly or in connected groups, may in turn contain liquid or solid inclusions. In some cases, both liquid and solid inclusions are found in the same void. Furthermore, genuine emerald may contain flat, plate-like inclusions which can he identified as calcite. Minute, well-developed crystals of pyrite are also sometimes found in real emerald. It is very, very rare indeed that an emerald of purest color is found as an absolutely flawless gem. When such a jewel does appear in the jewelry trade a price of five figures per carat is generally asked. In the case of synthetic emeralds, especially those of poorer quality, a series of wisp-like inclusions are always present. They are extremely characteristic of this material. However, within the past year synthetic emeralds have been examined that proved to he quite clear, containing relatively few of those inclusions that were so common in earlier German and American synthetics. Where the quality is good a very close microscopic examination of the inclusious is necessary. Naturally, if the microscope reveals small, well-defined crystals of pyrite, or plates of calcite, the gem is unquestionably genuine. Furthermore, synthetic emerald does not contain the so-called three-phase inclusions referred to above, of the kind common to the natural mineral. (3) Indices of Refraction. A refractometric examination of American synthetic emerald gave readings of 1.56Er1.570. I. G. Farben "Igmerald" was found to have indices of 1.561-1.564 (14, 15). Genuine emerald hasslightly higher refractive indices, 1.569-1.577 for the Chivor emerald, and 1.57G1.580for the finer Muzo varieties. To determine those close differences in refractive indices it is essential to use monochromatic light. (4) Specific Gravity. When the stone is not mounted density is always a valuable test. It is particularly useful in the case of separating synthetic from real emerald. In manufactured emerald, one finds the specific gravity to he 2.65 for the Chatham product and 2.67 for "Igmerald." Genuine emerald has a density of 2.70, the finest or purest gems have 2.71. (5) F l u o r e s ~ e .A quick and fairly sure way to tell a synthetic emerald from a real one has been by the presence or absence of fluorescence under ultraviolet light or X-ray irradiation. With a filter designed to transmit 3600 A. and a good quartz ultraviolet lamp, synthetic emerald reveals a maroon fluorescence; genuine Muzo emerald does not. X-rays will yield the same result-positive for the synthetic, negative for the real. However, Chivor emeralds also reveal a maroon fluorescence, although somewhat weaker than that of synthetic emerald. This test is not as important as it once was, but it is still valid if the gem under examination is of Muzo origin. (6) The Emerald Filter. For many years there has been used in the jewelry trade an absorption filter, known as an "emerald glass." With this simple device a real emerald appears pink or red on examination hefore a strong light source. On the other hand, any green mineral, such as a tourmaline, for example, or a
Photo by Fred Lym for Coliien
piece of green bottle glass, remains green through the filter. Wit,h the introduction of synthetic emeralds it. was found that these, too, appeared red through the filter glass. While the synthetic material is usually a brighter red than the genuine further examination must he made to determine whether the emerald is real.
(1) ALEX~NDER, A. E., J. CHEM.EDUC.,23,418 (1946). .4.E., J . Sediment. Petrol., 4, 12 (1934). (2) ALEXANDER, (3) ALEXANDER, 8. E., Jewelry Magazine, 64 (Aug. 15, 1948). (4) LIDDICOAT, R. T., G e m end Geml., 5,485,504 (1947). A. E., Ganmologist, 16,307 (1947). (5) ALEXANDER, P., Compt. rend. sei., 106,1800 (1888). (6) HAUTE~UILLE, ( 7 ) SCHIEBOLD, E., Zeit. Kryst., 92,435 (1935). A. E., Jewelry Magazine, 41 (Oct. 15,1947). (8) ALEXANDER, (9) POUGH,F., Jewelers' Circular-Keystone, 176, 224 (Dec. 1947). E . J . , J . Gemmol., 1, 7 (1948). (10) GUBELIN, G. VAN,Geol. Mag., 84.98 (1947). (11) PRAAGH, ~ , Atti. reale acead. sci. Torino, 41, 158 (1906). (12) S P E Z G., N.. AND W. A. WOOSTER, Nature, 157,297 (1946). (13) WOOSTER. (14) S m m , G. F. H., "Gemstones," Methuen & Comprtny, London, 1940. (15) ANDERSON, B. W., "Gem Testing," Heymood & Company, Ltd., London, 1947. (16) Bell Lab. R e d , 26,384 (1948). A. E., Jewelry Magazine, 59 (Mar. 15,1948). (17) ALEXANDER, (18) bid., 65 (April 15, 1948).
