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.

 

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