Topaz




Topaz

Topaz is the orthorhombic fluosilicate of aluminum. The name itself, according to Pliny, was derived from Topazios, an island in the Red Sea that was probably the early source of peridot. The island was named for a Greek world meaning "to gues", because it was often obscured by fog and difficult to find. Some present day authorities believe that the name is a derivative of the Sanskrit word "topas" meaning "fire."

Perhaps no other gem has had more varied or preposterous powers ascribed to it. When worn as an amulet, it was said to drive away sadness, strengthen the intellect, and bestow courage. A topaz mounted in gold and hung around the neck was believed to dispel enchantment. When the powdered stone was put in wine, it was used as a cure for asthma, insomnia, burns and hemorrhage. It was considered the stone of fruitfulness and faithfulness, conferring cheerfulness on the wearer, calming passions, and preventing bad dreams. Topaz was supposed to become obscure in contact with poison and quench the heat of boiling water. All of these powers were believed to increase and decrease with changes of the moon.

In the Indian gem trade it is seen that the terminology for "Topaz" is misused on a large scale. When Quartz is sold as "Golden, lemon or smoky Topaz". With the use of these terms the importance of actual topaz is not at all felt. Using these terms may be unfair trade practice.

Varieties and trade names of Topaz

Unlike many of the other gemstones, there are no distinct well known variety terms applied to the various colors of topaz, even though it is available in a broad range of colors. Until somewhat recently, most of the varieties were known by incorrect terms that compared topaz with better known gem varieties and species. For example, such terms as "Brazilian sapphire" and "Slaves diamond" were used for topaz that resembled the color of these two gems. Fortunately, these and similar terms have passed into antiquity; they are rarely used today.

Brownish-Yellow to Yellow-Brown Topaz

The most important variety is that frequently referred to in the trade as sherry topaz. The reason, obviously, is the resemblance of its color to that of sherry wine. The appearance of sherry topaz is often called "velvety"; it has a "softer" quality than that seen in citrine quartz, which is so frequently sold as topaz. A dark-orange to dark orange-red color is sometimes called hyacinth topaz, but it is not a common term nor a commonly encountered variety. A slightly orangey yellow to yellow or golden-yellow topaz is found in Brazil and is often called Brazilian topaz; however, this name could be applied to any variety of topaz found in that country. It is frequently used with the more or less sherry-colored topaz, as well as the golden yellow.

Light Blue to Very Light Blue Topaz

Blue topaz has been found in fairly substantial quantities in Brazil. Frequently, it is found in parcels of aquamarine shipped from Brazil to various gem centers. The rough is usually found in comparatively small sizes, but an occasional very large stone reaches the market. One stone of over 3000 carats was cut by a Los Angeles colored-stone dealer, more as a curiosity than as a gemstone. The dimensions of the stone were a matter of inches in length, width and depth; the length was in excess of three inches. Although blue topaz is generally very light in tone, exceedingly attractive medium-light stones are sometimes found; of course, they command a significantly higher price. Blue topaz is also found in the Kanta Banji Area of Orissa.

Pink Topaz

Light-red to light-violetish-red stones are usually sold as pink topaz. This variety is rarely found in this color in nature, but usually is produced by heating the sherry, brownish-yellow topaz without the use of chemicals. This process is referred to as "pinking" and stones produced in this manner sometimes are called "pinked" topaz. Although pink topaz is a lovely stone in its deeper tones, it is very rarely seen in a jeweler's stock it is one of those overlooked beauties among the colored stones.

Colorless Topaz

Another fairly well-known variety is the colorless or so called white topaz. In the past, this stone has been referred to, with a prefix, as a diamond; i.e., "slaves diamond." Its used in jewelry is limited to either inexpensive pieces or to the substitution for diamonds in calibre work or other pieces utilizing small stones. Its value as a gemstone is limited greatly by its comparatively low dispersion and brilliancy; in other words, for topaz to be desirable as a gemstone, it must be valued for the beauty of its color. It is also found in the Kanta Banji Area of Orissa.

Other Varieties

Topaz is occasionally found in a number of other colors. Among these are light violet, red of medium tone, and pale greenish yellow to yellowish green. These varieties are either too rare or too little in demand to be encountered frequently by jewelers. If the depth of color of the various varieties of topaz were greater, there is little doubt but that it would be numbered among the most important of the gemstones. It has the durability, a very pleasing appearance otherwise, and it is effective as a ring stone. The pastel colors in which it does occur are so attractive to many persons that topaz does offer significant possibilities to the colored stone man.

Formation of Topaz

Topaz is formed by pneumatolytic processes; i.e. those in which the action of hot gasses plays an important part. Usually, it is found as a constituent of pegmatite dikes, where, as indicated by the descriptions given earlier, the crystals often attain huge size. It also occurs in cavities in highly acid rocks, such as rhyolite, and in gneisses and schists. Because of its hardness and relative durability, it is a common constituent in gem gravels.

Topaz is one of the materials formed late in the cooling of a silica rich igneous mass. The more volatile constituents left and concentrated by the deposition of the materials that crystallize early may act on either the already crystallized materials or on the surrounding country rock. Products of pneumatolytic action, in addition to topaz, include cassiterite (tin oxide), tourmaline and apatite. One of the important constituents of topaz is fluorine, which is partly replaced in some cases by the hydroxyl (OH) ion. Both fluorine and OH are among the volatile materials concentrated by the crystallization of other components of a silica-rich magma. It is probable that the fluorine content of topaz is responsible for its rather low R.I. in relation to its specific gravity. Most fluorine minerals are relatively low in refractive index.

Sources of Topaz

Although topaz is far from common in sizes suitable for gem use, It is a mineral of wide-spread occurrence. It is found in other than gem quality in many countries and in many kinds of deposits. In most of the areas in which topaz is mined as a gem material, it is either a by-product of the mining of other gems or at least not the sole material being sought in the mining operation. It is one of the many gem quality minerals found in the gem gravels of Ceylon and in the Mogok area of Burma. In these two gem rich areas, it is mined as a by product in the search for ruby and sapphire. In the rich gem sources that are the pegmatite dikes of Brazil, topaz is likewise mined; here, however, it is one of the targets of the gem miners exploitation of the mines along with beryl. These same pegmatites are often mined for such mineral wealth as tungsten, columbium and tantalum. Beryllium and lithium are other metals that may be sought by the miners. In deposits in which the topaz is disseminated in cavities of a rock such as rhyolite, it is mined by blasting the rock and searching for the cavities in the pieces broken out by the blast. There are times when this tends to fracture or cleave the topaz, but usually enough cleavage-free pieces are obtained to make this process worth while, if they are handled carefully.

The most important pegmatite-dike source of topaz is Minas Geraes, Brazil, near the town of Rodrigua Silva. Brazil produces stones ranging from yellow to dark sherry, as well as pink, blue, pale green and colorless. Blue, green and colorless material comes from Ceylon. In general, Ceylon material is not a very deep color. Fine blue, stones have been found in the Ural Mountains of Russia, but, of course, this is not a source of any commercial significance today. The Kanta Banji area of Orissa has a untapped Huge deposit of Topaz of good quality going up to the weight of about 4000 to 6000 carats. These area found associated with Emeralds in pegmatites.

Topaz Jewelry

Topaz was formerly cut in the round brilliant and mixed styles, often with many additional facets. Today, however, step and modern diamond styles are used primarily.

Since topaz has an excellent basal cleavage, it must be oriented very carefully, because difficulty in grinding and polishing will be encountered if the table is placed parallel to the cleavage plane. Moreover, if the girdle is positioned parallel to the cleavage plane, numerous small cleavages are apt to occur on the girdle edge. Usually, orienting the table 12° to 15° off the basal cleavage plane or off the length of the crystal will eliminate this difficulty.

Topaz is not overly sensitive to the heat generated during any phase of the fashioning operation; therefore, only the usual precautions are necessary. Facet grinding is accomplished on a diamond impregnated copper lap or a diamond bonded plastic lap, and the most satisfactory polish is obtained with Linde A powder on a tin lap. Topaz usually polishes easily and requires no special techniques; however, if a facet seems difficult to polish, permitting the lap to run almost dry will produce the desired results. Polishing on a nearly dry lap must be done with caution, because the heat thus generated may loosen the stone on the dop. Suggested angles are 43° for the main crown facets and 39° for the main pavilion facets.

Physical & Optical Properties of Topaz

Physical Properties

Chemical Composition A fluosilicate of aluminum, expressed by the formula Al2(F,OH)2SiO4.
Crystallographic Character Orthorhombic system. Habit: prismatic, often with pyramidal terminations. The accompanying photograph shows a blue topaz crystal (L) and a stream-worn pebble (R).
Hardness 8
Toughness Poor. Dropping on a hard surface, or even light blows, may damage a stone badly.
Cleavage Highly perfect parallel to the base of the crystal.
Fracture Conchoidal
Specific Gravity 3.49 to 3.57. Yellow to pink usually 3.52 to 3.53; white and blue, 3.56 to 3.57
Streak White
Characteristic Inclusions Cavities containing two non miscible liquids and a gas bubble.

Optical Properties

Degree of Transparency Transparent
Luster Vitreous
Refractive Index Colorless, blue and green; 1.609 - 1.617. Yellow, brown and red: 1.629-1.637.
Birefringence .008
Optic Character Biaxial positive
Pleochroism Weak to distinct.
Blue: colorless and light blue.
Yellow: brownish yellow, yellow and orange-yellow.
Brown: yellow-brown and brown.
Red and pink: light red and yellowish red to yellow.
Green: blue-green and light green.
Dispersion 0.14
Phenomena None
X-Ray, Fluorescence Blue and colorless stones glow a greenish or violet-blue color. The color emitted by yellow ,brown and pink stones varies from brownish yellow to orange.
Ultraviolet Fluorescence Colorless: none to pale yellow.
Red: weak brownish yellow.
Yellow: weak orange-yellow.
Blue: none. Brown
Absorption Spectra Non diagnostic spectrum.

Effects Caused by:

Heat Although topaz is infusible under the Jeweler's torch or the blowpipe, it may lose or change color entirely. Too rapid heating or cooling will cause internal cracks or possibly cleavage.
Acids Affected very slightly.
Irradiation No data available.

Test and Identification of Topaz

Topaz lies in the "in-between" area of refractive indices, where there are to be found many of the unusual gemstones. As a result, one must exercise some care in testing for topaz. In addition, the properties of glass may be substantially the same, although it is not doubly refractive.

Undoubtedly, the gemstone most frequently confused with topaz in testing by the inexperienced tester is tourmaline. Tourmaline has refractive indices in the range of those of topaz, but it is sufficiently higher in birefringence so that there should be no difficulty in separating the two. In addition, tourmaline has a much lower S.G. The heavy liquid, methylene iodide (density, 3.32), will separate the two quickly, for tourmaline floats and topaz sinks. An examination under magnification discloses strong doubling in tourmaline and weak doubling in topaz. The birefringence of tourmaline is more than twice that of topaz, with topaz showing refractive indices either in the 1.609-1.617 range or the 1.629-1.637 region, whereas tourmaline has values near 1.624-1.644. Thus, the birefringence of topaz is only .008, compared to .020 for tourmaline. Tourmaline is usually strongly dichroic, but only in the pink color is topaz noticeably dichroic. Most tourmaline is considerably darker in color than corresponding varieties of topaz, but some pink and sherry topaz may be deeper in tone than some tourmaline. Regardless of color, however, it is a simple matter to separate the two on the basis of birefringence, either in the form of doubling of the back facets or on the refractometer, or by the difference in S.G.

Glass that has been compounded to make its R.I. and S.G. approximate those of topaz is difficult to distinguish, except by the polariscope. In addition, glass shows no birefringence or doubling. By rotating a Polaroid plate before the eyepiece of a refractometer, the reading obtained from the glass will stay the same in all positions; conversely, both indices of topaz will become apparent, if the stone is being tested in other than a direction of single refraction. Usually, glass that imitates topaz has much too much dispersion for comparable topaz. A few years ago, a huge topaz was offered for sale in the trade of approximately $8,000. It was a lovely, dark-sherry color, considerable interest in the stone. An old-time gemologist was shown the stone, and despite the fact that the R.I. was 1.62 and the S.G. 3.53, he suspected its identity it did not look "right" to him, because the fire seemed too strong. The use of the polariscope immediately ruled out topaz as a possibility.

The rarer gemstones that resemble topaz in properties include danburite, andalusite and apatite. Each of these materials floats in methylene iodide, whereas topaz sinks; thus, a specific-gravity determination by heavy liquids is adequate to a separate any one of the three from topaz. Apatite may also be distinguished by resolving an interference figure, since it is uniaxial. If the stone being tested is mounted, R.I. readings using a monochromatic light source obtained by lamp or filter might be needed to separate the other possibilities. Danburite shows indices of 1.630 and 1.636 both varying from the mid point (beta index) at 1.633. Colorless topaz shows indices at 1.609-1.617, but the yellow reads at 1.629-1.637. However, the midpoint (beta index) is only .001 from the low reading; thus, the low reading is virtually constant.

Frequently, topaz is substituted for aquamarine. On occasion, identifications are made incorrectly between these two merely because the refractive indices are approximately two units from the prominently marked 1.60 line on the scale of any refractometer. It is not difficult, when one is in a hurry, to mistake a 1.62 reading of topaz for a 1.58 reading of aquamarine. This happens when an inexperienced tester looks at a stone, assumes it to be aquamarine, and thus expects the R.I. to be 1.58. A quick glance at the refractometer scale where a reading appears two points away from 1.60 is carelessly read frequently as 1.58, rather than 1.62.

One of the characteristics of many topazes under magnification is fairly large liquid and gas filled cavities containing two nonmiscible liquids in which gas bubbles in the enclosed fluid appear to have two distinct edges (not caused by doubling). This effect is illustrated in that part of the assignment dealing with physical and optical characteristics. Another characteristic that is sometimes visible under magnification is basal cleavage; in other words, topaz shows one direction of excellent cleavage, which is often evident in a stone with a number of flaws.

Valuation of Topaz

A lack of knowledge on the part of the public of the existence of the various colors of topaz, other than yellow, has affected the value of gems of great merit and beauty. Its cleavage is sufficiently easy that it is not too wise a choice for a man's ring, unless the style of mounting affords it adequate protection. Most specimens exhibit an exquisite and unusual body appearance. Feathers, flaws (often consisting of liquid or gaseous inclusions), opaque inclusions and poor color are frequent. Clean, fine quality gems are not common. Yellow Russian topaz is subject to fading. Blue stones are unusually beautiful, some rare specimens resembling closely the color of sapphires of the trade grade known as Ceylon sapphire. With rare exceptions, all colors, except yellows and browns, are light to very light (pale) in tone. Other factors being equal, the darker the tone, the more valuable the stone. Pure yellow topaz is encountered infrequenty, and even then the value is comparatively low, since this color may be duplicated by the very inexpensive citrine quartz. The violetish-red stones in fine qualities are the most expensive, followed by pink, sherry and related colors, and then blue and yellow. The more valuable topaz in any color displays a slightly velvety body texture in conjunction with a high, almost sub adamantine, luster.