Turquoise

Turquoise




Four bracelets made of turquoise and cast gold are the oldest pieces of wrought jewelry known in the world. For seventy-five thousand years they remained on the arm of the mummy of Queen Zer, an Egyptian queen, and were still beautiful when they were excavated in 1900. Today, alternating with zircon as the December birthstone, turquoise enjoys the same high degree of popularity as it did in the ancient world; in fact, it is often classed as the most important of the opaque gem stones.

The name turquoise (pronounced TUR-kwoyz), which was first used in the thirteenth century, is though to have come from a French word meaning "Turkish stone" (pierre turquois), probably because the gemstone first reached Europe by way of Turkey. The Persian word for turquoise is "ferozah" or "firozah", which means "victorious". In Tibet, turquoise is called "gyu", curiously similar to the term "yu" which the Chinese apply to jade, their most precious gem. Turquoise is often spelled with an "e" as the final letter, but in this country mineralogists agreed to be consistent and drop the "e".

The history and romance associated with turquoise is exceeded by that attached to few other gemstones. Lengthy accounts of legents and superstitions are contained in almost every book dealing with the subject of gems and jewelry. Most ancient civilizations valued turquoise very highly; the Aztecs, Incas, Persians, Egyptians and the southwestern American Indians are those most frequently mentioned. In Tibet today it is by far the most popular of all materials used for personal adornment, and it plays an important part in the religious ceremonies of the Tibetan people. During the sixteenth century, turquoise was used as a medium of exchange by the Indians of the southwestern United States. They used the stone to adorn their house fronts, their graves and their persons to them, it embodied the spirits of the sea and the blue sky. Warriors, many animals to the hunter, and happiness and good fortune to all who wore it. Another widespread belief was that turquoise had the power to protect the wearer from injury by falling, especially from horse back, and that it made the horse more sure footed. A belief still prevalent among the Navajos is that a piece of turquoise thrown into a river with prayer to the rain god will almost surely bring rain shortly.

To date, the oldest piece of turquoise jewelry made by these early American Indians was found in Death Canyon, Arizona; it is a pendant consisting of a hardwood ring, three and one-fourth inches across, bearing a mosaic formed of eighty one pieces of turquoise affixed to the wood with gum. Even today, the American Indian still prizes turquoise highly among the Navajos, no Indian is likely to be seen without his piece of turquoise; the higher his position in tribal society, the finer his stone. Few religious rites of the Indians of New Mexico and Arizona take place without turquoise. Almost all of the Indian jewelry in the American South-west is the turquoise set type. Although this has been commercialized increasingly in recent years, the importance of turquoise to the tribes of the Southwest was no less centuries ago than it would seem to the casual observer today.

All of the cherished gem stones of earlier civilizations are surrounded by fascinating lore. Although all of the gemstones were regarded as bearers of good fortune to their owners, the wealth of beliefs about the benefits of owning turquoise is hard to match.

Varieties of Turquoise

The copper content of turquoise is responsible for its blue color, and the presence of iron accounts for a common greenish cast. Since the copper is an essential constituent and iron is also regarded as such by many, turquoise is classed as an idiochromatic stone. The following grading terms have been used in the trade to indicate the quality of turquoise, but they do not necessarily denote geographical origin:

  1. Persian
    This variety is intense medium blue and is the least porous of the turquoises; therefore, it has a slightly higher specific gravity than most material and takes a much better polish.
  2. American or Mexican
    Pale blue to light blue, as well as greenish blue to bluish green, characterize this type. In addition, it is often somewhat porous.
  3. Egyptian
    The variety of turquoise called Egyptian is greenish blue to yellowish green. Although it usually has a lower porosity and a higher density than American turquoise, the color is poorer, because of a higher iron content.
  4. Turquoise Matrix
    Specimens containing portions of the mother rock are called turquoise matrix. Stones with included matrix resembling a spider web pattern are the most important of this type.

Formation & Sources of Turquoise

Turquoise is a secondary mineral. It is deposited in veins and as nodule (rounded masses) near the surface by circulating ground waters carrying copper, aluminum and phosphorus leached from earlier rocks. The tabular deposits are seldom sufficiently wide to produce thick stones. The fissures are often so narrow and irregular that the mineral recovered can be used only as turquoise matrix. It is found occasionally in alluvial deposits as nodules, but it weathers fairly rapidly when exposed at the surface.

Sources and recovery methods

(Note : The numbers in parentheses in this portion of the text refer to approximate locations of sources on the accompanying maps.)

The ancient mines near Nishapur, in the northeastern corner of Iran (Persia), about fifteen miles west of the town of Meshed, are usually considered to be the source of the finest turquoise. Before World War I, turquoise mining was a leading industry in Persia, providing the government with a substantial annual revenue, but production today is at a virtual stand-still. For hundreds of years nearly western Asia came from this famous source, and for many decades it supplied the American trade as well. The entire deposit lies on the southern slope of Ali-Mirsai, one of the peaks of the mountain range in the region. The turquoise occurs both in veins in an altered trachyte (pronounced TRACK-ite), which is an extrusive igneous rock, and in alluvial deposits. The finest stones are found in the alluvial source, where they occur as pebbles weathered to a chalky-white material on the outside but containing cores of fine quality turquoise. Other Persian deposits are situated near Kerman and Yezd these, however, have never been important producers.

The oldest turquoise mines in the world are situated on the Sinai Peninsula, on the north side of a valley called Maghara Wadi. Here the gem is disseminated in thin seams through a red iron bearing sandstone, as well as in a porphyritic igneous rock. As early as 5500 B.C. (before the first dynasty), the Egyptians used turquoise for personal adornment, as proved by beads found in prehistoric graves antedating the dynasty period. By the time of the first dynasty, about 3200 B.C.), the kings of Egypt were sending expeditions of miners to Sinai. These royal mining expeditions were highly organized, often comprising two thousand or three thousand laborers with a military escort. The long journeys were a usually begun in November and concluded in May, before the summer heat became excessive. In at least one instance, however, three years elapsed before the adventurers returned home. The mines were worked in this manner over a period of approximately two thousand years, until late in the twentieth dynasty (about 1100 B.C). The quantity of turquoise obtained was never very great, although historical records show that about 882 pounds were produced in one year. Malachite was a by product, and the miners usually brought back manganese and copper ores for use chiefly in glazes and enamels.

In the nineteenth century, about 1845, the old mines at Wadi Maghara were discovered by an Englishman and worked on an extensive scale from about 1854 to 1866, when operations were discontinued. Some what later a Frenchman worked the mines {or a short period, and in 1903 English promoters obtained a concession. This last attempt, however, was a complete failure, and work was abandoned after a year or two. Even now, occasional stones are picked up in this region by the Bedouins, who barter them in exchange for articles they need; but all known deposits on the Sinai Peninsula are completely exhausted, and there is little reason to believe that further discoveries will be made that will lead to a revival of systematic mining.

Other foreign sources of little or no importance are Tibet, China, Australia (Queensland, Victoria and New South Wales), Mexico (Sonora and Zacatecas), Peru, Chile, Turkestan and Afghanistan.

Today, the United States is the world leader in the production of turquoise, most of which comes from the southwestern states. Occurrences in Arizona are near Tombstone, India.

There are reports of Turquoise from Tibet and Andhra Pradesh in Mining is usually a very simple process that requires little subsurface shaft sinking and tunneling, for turquoise is never found in quantity at depths exceeding one hundred feet. Usually, it is exposed on the surface, and good material is often at a depth of but a few feet. An open cut trench or shallow pit suffices for working the shallower deposits, but for extensive operations at slightly greater depths a shaft is usually sunk and tunnels extended horizontally there from at intervals along the vein. Most turquoise deposits are found in arid reigns, where drainage is no problem. This, plus the limited extent of most deposits and their shallow depth, usually warrants little in the way of expensive or elaborate equipment. Thus, most turquoise mining is rather primitive. After the rock has been loosened, it is broken by hammers and raised to the surface in buckets hoisted by rope and windlass. Additional crushing is then done, after which it is hand picked for rejection of unsuitable material.

Physical & Optical Properties of Turquoise

Physical Properties

Chemical Composition H5(Al(OH)2)6CuOH(PO)4)4 A complex hydrous copper-aluminum phosphate. Iron may replace some aluminum. Copper causes the blue color; iron may cause .he green color. Water content also influences the blue color.
Crystallographic Character Triclinic system; cryptocrystalline. Minute crystals are known to occur, but they are rare
Hardness 5 to 6
Toughness Chalky material is poor and easily fractured; fine quality is fair to good.
Cleavage None
Fracture Conchoidal, granular
Specific Gravity 2.61 to 2.84; normal, 2.76
Streak White or greenish
Characteristic Inclusions Matrix in the form of limonite or other iron oxides.

Optical Properties

Degree of Transparency Opaque; thin sections are translucent
Luster Polished surfaces are waxy to vitreous; fracture surfaces are waxy to dull.
Refractive index 1.61 to 1.65 (doubly refractive). Because turquoise is an aggregate, only one reading is visible on the refractometer, usually at about 1.60.
Birefringence Strong, but not apparent by any gemological test.
Optic Character Since turquoise is opaque and cryptocrystalline, its optic charioteer cannot be proved by usual gemological tests. Crystals are biaxial, positive.
Pleochroism None
Dispersion None
Phenomena None
X-Ray Fluorescence None
Transparency to X-Rays Nearly transparent
Ultraviolet Fluorescence None to weak greenish yellow under long wavelength; inert under short wavelength
Color-Filter Reaction None
Absorption Spectra Two hardly visible bands at 4300 and 4200 A.U. are occasionally seen in strong reflected light.
Effects Caused by heat Usually decrepitates (i.e., pieces "explode" off the main mass until nothing remains), turns brown and colors the flame green.
Effects Caused by Acids Dissolves in Hydrochloric acid.
Effects Caused by irradiation No effect

Test and Identification of Turquoise

Usually, turquoise can be recognized by its color, opacity and luster. It occurs only in light to medium tones of blue and green, colors that are characteristic of very few other semi-translucent to opaque stones. Most turquoise has a chalky appearance in the rough, a sub vitreous luster at best when polished, and is opaque. Fine blue specimens have a waxy luster in the rough, are vitreous on polished surfaces, and are semi translucent. On the basis of a word description alone, turquoise might be confused with many non transparent materials, but when descriptions of body appearance and transparency are added, there are very few stones that bear a close enough resemblance to any type of turquoise to cause difficulty.

The appearance of blue turquoise may be somewhat similar to chrysocolla quartz (chalcedony), odontolite and glass. Although fine-quality chrysocolla quartz is almost identical in appearance to the best turquoise, it is usually well polished and can therefore be separated readily by R.I. Also, it lacks the chalky appearance that often characterizes turquoise. The mineral chrysocolla itself resembles turquoise, but is readily distinguished by its low S.G. (2.24 or lower) and a hardness of a 4 or less. Odontolite not only has a higher density than turquoise, but it shows the typical cellular structure of bone under magnification. (Note: Heavy liquids for making S.G. determinations should be avoided, for bromoform and methylene iodide may discolor highly porous turquoise.) Glass imitations frequently show swirls of color (blue and white streaks) on the base, or the base is rough ground, as opposed to the polished base of most turquoise, There may of mold marks around the girdle or hemispherical cavities on the surface, which prove the presence of gas bubbles. Glass usually has a somewhat lower R.I. (about 1.58), although occasionally it might he the same as turquoise (1.60). It should be noted, however, that an index taken of a curved turquoise surface by the spot method will often be lower (1.58 or 1.59) than the reading expected from a flat surface. If possible, the reading should be taken on the flat back surface, in which case the normal 1.60 index will be obtained. A highly distinctive feature of the majority of glass imitations is that they have a vitreous luster on fractures, whereas the fracture luster of chalky-blue, turquoise is dull and that of the finer, semi translucent, solid stones is waxy. Under magnification, bubbles can sometimes be seen just below the surface of glass. Remember, too, that matrix inclusions can be imitated very well in glass; however, an examination of this "matrix" will disclose it to be "flush" (even) with the surface. Matrix in turquoise, since it is either harder or softer than the body of the stone, will usually be revealed as depressions or protrusions.

The green varieties of turquoise can be confused only with a few stones that have a similar body appearance; these are variscite, malachite, and greenish odontolite and chrysocolla quartz. (The latter two can be separated by the means discussed above; in addition, odontolite is rarely greenish). Variscite has a lower R.I. and S.G. than turquoise and usually occurs with a bright-yellow mineral that is never found in association with turquoise. The malachite used for gem purposes has an agate like color banding in various shades of dark and medium green; too, its S.G. and R.I. are much higher.

Additional cautions to be observed in the identification of turquoise are concerned with the polariscope and streak tests. Even a turquoise that transmits enough light around its edges to suggest analysis in the polariscope is unlikely to give satisfactory results. Not only the turquoise, but the rough-backed glass imitation as well (the most common substitute), is likely to remain light at the edges when rotated in the dark position of this instrument. Because of the softness of many pieces of turquoise, streak tests should be limited to the girdle edge, to avoid damage to the polished surface. In general, a streak test should be avoided.

Since a knowledge of artificial substitutes and treated turquoise is of importance to the Jeweler. It is necessary that these materials also be given careful consideration. Several years ago, an artificial substance that closely resembled blue turquoise, both in appearance and properties, was produced on a commercial scale. This substitute could be detected by immersion in water for a few hours, which caused it to become a darker blue with a network of fine, lighter colored lines (almost white) on the surface; it also became softened when immersed in alcohol. More recently, a plastic-bonded substitute was placed on the market that resembles the spider-web variety of blue turquoise. It is produced by the precipitation of an artificial compound that is similar in composition to turquoise, When the precipitate hardens, it is broken into small pieces and bonded together with plastic under low pressure. The dark lines in this material indicate the plastic bond and provide the best means of detection. Furthermore, the softness of the plastic can be determined with a sharp needle, whereas the dark matrix in turquoise is harder and will resist the needle.

Because of its porous nature, the color of turquoise can be improved by impregnation with dyes and other materials. This is usually accomplished with the aid of low heat and or pressure. The oldest form of treatment is soaking in oil. Another method is to boil the stone in paraffin or some another kind of wax. Either method causes a darkening of the color and makes the stone more translucent (less chalky looking). However, the improvement is only temporary and actually seems to hasten the gradual change (which is common to most turquoise) to an undesirable green color within a few months. A more recent method of treatment is impregnation with sodium silicate (water glass) or various kinds of plastics, which produces an improvement similar to that of oil or wax. Stones treated with these materials hold their color longer than those treated with oil or wax. Many kinds of dye have also been used, but none has proves entirely satisfactory; most of them produce an unnatural color or are not permanent.

Unfortunately, the best method of detecting oil and paraffin treated stones reduces their beauty; however, it does reveal their true nature. The method consists of soaking the suspected stone overnight in carbon tetrachloride, during which time all of the oil or paraffin will be forced out of the stone, returning it to its original (pretreated) appearance. If placed in the sun or heated gently for a short period of time, some oiled or waxed specimens will "sweat" the impregnated substance to the surface. If they are placed in hot water just below the boiling point, a film of oil or wax will usually appear on the water's surface. Others, however, are coated with a plastic material that prevents the oil or wax from reaching the surface. Stones impregnated with sodium silicate or plastics can be detected only by the use of destructive tests.

In a jewelry trade magazine it was suggested that a drop of hot water on the surface of untreated turquoise would be absorbed immediately but would stand up on the surface of a treated stone. This is true only of the very chalkiest, untreated material that is unfit for gem use, and even then it is far from in fail. Actual tests have shown that the rate of evaporation of the hot water on untreated turquoise is quite rapid (but consistent with the action of water at this temperature on an impervious surface) and thus creates a false impression that it is soaking into the stone.

Greenish stones that have been colored by blue aniline dye will turn greenish or white when tested with a drop of ammonia. Other dyed stones can be detected only by a deep scratch; the color in the scratch will be much lighter than the outside color. However, some dyed stones cannot be detected by any method short of chemical analysis.

Most turquoise now on the market has been treated by one or more of the afore mentioned methods. Many of the tests necessary to determine the nature of the treatment are not practical for use on a polished stone. Since the most important consideration for the jeweler is to avoid selling stones that might fade or discolor readily, it is wise into insist on a written statement from the supplier guaranteeing that the stone he sells will retain their color for at least a specified period of time.

Valuation and Buying Tips for Turquoise

Valuation

Basically, there are two types of turquoise: that without matrix and that containing varying amounts of matrix. The term spider web turquoise indicates an otherwise smoothly colored specimen, except that the surface is covered with a rather evenly distributed network of very fine lines. If these lines are not thread like in fineness, the stone is merely considered as turquoise matrix. The prices given below for the four principal qualities of turquoise are wholesale values based on 14 to 18 millimeter cabochons.

1st Quality

  1. Stones of top quality are intense medium blue. The color is smooth and evenly distributed (no spots of darker or lighter blue) and there is no trace of matrix, The high luster and semi translucency produce a "glassy" appearance on the surface. Such stones are usually priced from about $8 to $20 PER CARAT. This quality, which is generally known as Persian turquoise, is rare.
  2. Same as 1) except that the stone shows an attractive spider web effect across the entire surface. This quality, which is usually called Persian spider web, is priced at $4 to $12 PER CARAT.
  3. Same as 1) except that the stone contains varying amounts and patterns of matrix. Depending on the amount, color and location of the matrix (overall attractiveness being the principal consideration), prices vary from about $2 to $8 PER CARAT. This type is called Persian matrix.

2nd Quality

  1. This quality is light blue and not as intensely colored as Persian. In addition, when compared with Persian, these stones are more opaque. Although many approach semi translucency, the luster is not as high and the texture is often spotted with slightly lighter or darker areas. No trace of matrix is present. PER CARAT prices range from approximately $0.8 to $8.
  2. Same as 1) except that the stone shows a fine spider web pattern across the entire surface. Prices range from $0.5 to $4 PER CARAT.
  3. Same as 1) except that varying amounts of matrix are present. Prices range from about $0.3 to $4 PER CARAT.

3rd Quality

Greenish-blue to bluish-green stones, even in the finest quality, rarely exceed $16 for the entire stone. Poor-quality matrix stones often sell for as little as $5 PER STONE.

4th Quality

Stones in this category are light to dark yellowish green. The finest qualities are seldom worth more than $4 PER STONE; poor qualities with much matrix often bring no more than $2 PER STONE.

(Note: The qualities described in the second, third and fourth categories are frequently sold under such names as American, Egyptian, Afghanistan, etc. However, only the Persian quality is a strongly established trade grade. Actually, the present tendency is to consider all but the Persian quality as American turquoise) .

BUYING & SALES ASPECTS

Turquoise has an important potential value to jewelry stores of all sizes. Turquoise jewelry is available in a wide range of qualities: from the very inexpensive kind that is handcrafted by the Indians of the Southwest, to beautifully designed and Executed necklaces, bracelets, pins and rings containing the finest quality semi translucent stones that cost the jeweler several dollars a carat. The appeal of turquoise for the many who find it a lovely stone is based almost entirely on its pure, cool color. Light blue is the favorite color of many persons, and no other stone seems to exemplify that color as well as turquoise. Undoubtedly, an important factor in this appeal is the fact that it is comparatively inexpensive, but it does seem to compare favorably with some of the transparent blue stones from the universality of its attraction. Although the supply of fine-quality turquoise is limited, jewelry containing inexpensive varieties is available with some wholesale prices in Bombay. Because it is competitive in price to costume jewelry, it is one of several stones that is particularly suitable for introducing the idea of wearing genuine stones instead of imitations or synthetics. This is a most saleable item for tourists.

Synthetic Turquoise

Properties of Synthetic Turquoise:
Chemical Composition Copper aluminum phosphate hydroxide hydrate with traces of Ca, Si and Mg; no Fe
Hardness 4 1/2 - 4 3/4
Specific Gravity 2.56 - 2.75; closer 2.75
Transparency Semi translucent to opaque
Refractive Index 1.598
R.I. Range ± 0.01
Optical Characteristic Doubly refractive, but only a single reading seen on refractometer; opaque
Color Blue
Fluorescence X-Ray - None ; SW UV -Very faint yellow green; LW UV - Moderate greenish-blue
Spectrum None to very very weak
Inclusions No characteristic inclusions

TESTING :

Natural turquoise of the same intensity of color and properties shows an absorption spectrum whereas synthetic turquoise does not. Microspheres which are characteristic structural features of synthetic turquoise under high magnification are not present in natural turquoise, (Note: The so-called "synthetic turquoise" distributed by Gilem SA, Geneva, Switzerland, may not be a true synthetic turquoise because the tiny spheres are held together by a binding agent.)