Testing Properties

You have received instruction in and have practiced the use of each of the basic gem-tasting instruments. In page Table of Hardness, Specific Gravity, Refraction Index of Gemstones covering the properties of the important gemstones and many rarely encountered gem materials as well. At this stage of the training, you have not had enough instruction to be able to utilize the required equipment to maximum advantage. For example, in the next work project, which requires complete identifications, you will be using only stones that have flat facets, thus making it simple to take refractive index readings. The use of the refractometer for stones with curved or irregular polished surfaces be explained latter. However, a most of the gems that you will be called upon to identify in the store can be identified readily with the instruments now at your command.

How do we go about testing gemstones? In gem testing we use trained mind as a rapid data sorting system. Without conscious thought, most gemstones are eliminated as possibilities at a glance. The trainee gemologist, however, has neither the experience nor the training to eliminate stones in this automatic manner. He knows that turquoise never red, that malachite is never orange, and that coral is never green (although any one of them could be dyed to the colors mentioned), but he is not sure that any one of the allochromatic stones could not occur in a new color. With the instruments at hand, it is necessary to start with those that give results that provide the most rapid elimination. After a single test, or two at most, he is left with only a few possibilities.

The law officer who is seeking to identify human beings through the use of fingerprints uses about eleven points of comparison to classify them. Theoretically, this means that eleven to the eleventh power different sets of fingerprints could be identified by his indexing system. This is a number that approaches three hundred billion. The gemologist does not have a billion, a million or even a thousand different possibilities to consider. As a result; the number of tests upon which he must rely to separate the various possibilities is small. As soon as you have developed skill in the use of the required equipment according to the methods taught thus far, you will be qualified to make most of the identifications likely to be encountered in the average jewelry store. The number of gemstones and substitutes frequently seen in jewelry stores is under fifty. If the rarely encountered materials that are sometimes cut for gem purpose are added, the number of possibilities is not more than tripled. This list would include almost everything that has ever been cut, by even the most enthusiastic hobbyist-cutter. Even so, care is testing is necessary to be sure of an identification.

A large number of possibilities are eliminated by the experienced tester at a glance. He knows for example, that a stone that has an intense ruby-red, sapphire-blue or emerald-green color can, in each case, the one of only a few possibilities. He knows that a transparent stone in any color greatly limits the range of possibilities. In the first and second tests he wants to reduce the remaining list as rapidly as possible, so he can be certain that the unknown can be separated readily by the instruments available. On the other hand, hasty decisions based on misinformation or too little information cause errors. The gemologist has learned that new colors and new types of well-known gemstones appear occasionally; therefore, elimination .by appearance or magnification is made only when there is no possible doubt.

Unquestionably, the instrument that performs the most selective separation is the refractometer. However, as discussed earlier, we recommend that the first step after an initial unaided-eye examination be an examination under a loupe or a binocular microscope, such as the GIA Gemolite or the GIA Gem Detector. The unaided-eye examination often eliminates further need for testing, by disclosing the characteristics of manmade material. A conchoidal, vitreous fracture on the girdle of a stone that resembles turquoise eliminates this stone immediately; the only other possibility is glass. In this case, the unaided eye has given enough information to make an identification.

Under magnification, the sight of bubbles, curved lines or the dividing line between two parts of a doublet often is all the information the tester wants. In other words, he has proved the stone to be artificial. Knowing that, he may not even be interested in whether it is glass, a doublet or a synthetic. If he is accepting a ring or other article for repair, he merely lists ruby imitation or synthetic ruby, for example, on the repair slip and asks the customer to initial or otherwise acknowledge his statement that the stone is not genuine. If magnification discloses that it is natural or fails to provide conclusive evidence of identify, the next logical step is the use of the refractometer.

The Refractometer

As experience is gained in gem testing, the importance of exercising care In each of the tests will become obvious. For many identifications, just a very general idea of the stone's R.I., together with one or two other items of information, suffices; but you will soon learn how easy it is to be careless in reading the refractometer scale. For example, frequently 1.72 is mistaken for 1.77 and 1.58 for 1.62, by anticipating a reading at a point different from its actual position. Learn to be careful. Check every result as a matter of habit. Keep alert.

Magnification usually distinguishes the manmade from the natural, and the polariscope separates singly - from doubly-refractive materials. Specific-gravity determinations give numerical values, but this test not applicable to mounted stones. The greatest aid to the tester in narrowing the list of possibilities to a few at most is the refractometer. If you take full advantage of this instrument, the identification of most gemstones is made easy. However, for the very reason that makes the refractometer so valuable -- its selectivity among possibilities -- an error in reading is almost certain to result in an error in identification.

An examination of the accompanying R.I. table shows that the number of gems and substitutes at any one reading on the scale is very limited. If a second reading is obtained, an even narrower range of possibilities remains. The use of a filter or monochromatic light and rotation of the stone to obtain maximum separation of the two readings almost always means that the identification has been made, unless a synthetic of the same species is a possibility. The combination of R.I. and birefringence is a pair of high selectivity. There are several gems and substitutes that may have an index of 1.62 and several others at 1.64 but only tourmaline shows extremes of 1.62 and 1.64. Thus, tourmaline may be identified by the refractometer. On the other hand, a white-light reading at about 1.77 could be obtained on natural or synthetic corundum, garnet or from the crown of a garnet-and-glass doublet. A Polaroid plate rotated before the reading may cause it to jump back and forth from 1.76 to 1.77, which would eliminate the singly-refractive stones garnet and the garnet-and-glass doublet. However, magnification alone would identify the stone.

When two separate readings are seen, singly-refractive gemstones are eliminated. Furthermore, the additional information yielded is the most highly diagnostic obtainable in identification. The analysis of information obtained by observing the behavior of R.I. readings as a gem is rotated on the hemisphere.

In portions of the scale where no synthetic is to be found, glass, plastic and doublets must be eliminated as possibilities; but this is a simple matter, even if only one reading is seen in white light. Between lapis-lazuli and spinel in index there is no important natural singly- refractive stone, so the polariscope is used to eliminate glass and plastics.

Assembled stones can be identified or eliminated by immersion in a suitable liquid. Since glass imitations ore almost never above 1.69 nor below 1.44 in index, singly-refractive stones with readings between 1,69 and 1.81 must be spinel, synthetic spinel, garnet or assembled stones, and those below 1.44 must be opal or fluorite.

Further Testing Procedures

With the R.I. determination, the number of possibilities has been reduced to a handful. Depending on the R.I., the gemologist should now be able to judge what other steps are indicated to effect a positive identification. If the remaining possibilities include both singly - and doubly- refractive materials, the polariscope is indicated; it is also indicated if distinguishing between a single crystal of a doubly-refractive material and a crystalline aggregate . In the absence of a dichroscope, the polariscope also is used to detect pleochroism, if the presence or absence of pleochroism is decisive.

When it is necessary to make separations on the basis of S.G., either liquids or a diamond balance should be used. The gemologist uses this test is only when no other means is available to effect a separation, because it cannot be employed on mounted stones. On the other hand, ultraviolet lamps or the spectroscope are both favored because they do not offer this important drawback. The trainee needs to gain as much experience and practice as possible in the
use of each of the key instruments, so that the extra information he gathers will permit him to make identifications without having to remove stones from their mountings.

Following are a number of examples to show how the tester proceeds. The R.I. table with this assignment shows the common stones to be expected at any given R.I. reading, as well as the rarely encountered stones. At 1.62 the only commonly seen transparent stones are glass, plastics, assembled stones, topaz and tourmaline. If magnification discloses angular inclusions, the only possibilities are topaz and tourmaline. Reference to Chart I, which accompanied , shows how these two stones differ in properties. Tourmaline has a much ,has much greater birefringence. In a tranparent atone, this difference is detectable under magnification; it is also visible on the refractometer. On any facet at some point during rotation, tourmaline will exhibit its maximum birefringence of about .020. Further examination of the chart will show that the simplest means of separating these two species is by heavy liquids, since topaz sinks in the 3.32 liquid and tourmaline floats. Thus, the identification is simple. However, it is simple only if magnification has proved that the stone is doubly refractive. It is entirely possible for glass or plastics to have a 1.62 index, and, although plastic floats in 3.32, glass could sink.

In order to provide for positive identifications, it is necessary to get as much information as possible from each instrument. For example, under magnification the presence or absence of doubling should be noted, and on the refractometer any birefringence should be measured. If doubling is apparent or if two readings are seen on the refractometer, it is not necessary to use the polariscope to determine whether the stone is singly or doubly refractive, because this has already been proved. This does not necessarily eliminate the polariscope, however, the instrument is used to obtain the remaining information on optic character. How to determine optic character and sign will be taught later.

Suppose you are given a very small, light-yellowish-green stone to identify. Under loupe magnification no inclusions are visible, nor is doubling obvious. However, when the stone is put on the refractometer, readings are seen at approximately 1.65 and 1.69. What has been determined thus far? We know the stone's R.I. values and that it is strongly birefringent. Since it is very tiny, it is possible that the doubling would not be noticed by many testers under a loupe. But even a tiny stone with this much birefringence would have easily detectable doubling under a binocular microscope using 30x or less. It is unnecessary to use the polariscope, because we know that the stone is doubly refractive. An examination of Chart A shows that the only relatively common stone having this birefringence at this point on the scale is peridot. If you want to confirm the finding, it is necessary only to drop the stone in the 3.32 liquid, in which peridot sinks very slowly; on rare occasions it barely floats. Either way, it is obvious that the 3.7 is near 3.32 (usually 3.34).

Suppose you are testing a bright-green stone, noting what appears to be angular inclusions under a low-power loupe, and a 1.57 R.I. is obtained. The possibilities are thus reduced to emerald, glass, plastic or an assembled stone, according to the R.I. table. However, a white-light reading of 1.57 is too broad to eliminate synthetic emerald (1.561-1.564) safely on this basis alone. Unlike synthetic corundum and spinel, the inclusions in synthetic, emerald resemble those of the natural. Transparent plastic has such a lower S.G. that it is obvious when held in the hand, and it shows the characteristics of a molded product. These characteristics should permit easy separation from glass and substitutes. If the next step is to place the stone in the polariscope, it should be possible to either identify it as glass or eliminate glass as a possibility. In other words, if a stone with this index is singly refractive, it has to be glass. Glass was not eliminated on the basis of low magnification, because it is not unusual to see glass imitations of emerald containing angular and elongated fragments. These fragments retain their angular shape because they do not melt at the temperature at which glass melts. If the stone shows a doubly refractive reaction in the polariscope, glass is eliminated. In order to distinguish a triplet (two parts of beryl and a green cement) from emerald, it is necessary to put the stone in a liquid and determine whether it is a one-or two-piece stone. For this purpose, it is better to use water than one of the of the heavy liquids, which could attach the cement and mar the appearance. In order to distinguish between emerald and synthetic emerald, a more accurate R.I. reading, examination of S.G. by emerald liquid (2.67-2.68), and reaction to ultraviolet light all help (see Chart I for R.I. and S.C. comparison). The presence of three-phase inclusions proves natural origin. The other tests provide strong indications individually and proof as a group.

Assuming that you have only a loupe for magnification, you are given a colorless stone to identify. Even though studied carefully, no inclusions are visible. The R.I. is in the 1.76-1.77 range, the stone is doubly refractive in the polariscope, and it sinks in the 3.32 liquid. What is the next step ? The initial test is interpreted to mean that both natural and artificial materials are possibilities. The R.I. shows that the stone is either natural or synthetic corundum or a doublet. The polariscope would climate a garnet-and-glass doublet, because both parts would be singly refractive. Thus, we know that the unknown is either synthetic or natural colorless sapphire. How can one be distinguished from the other? The only possibility available, assuming that you do not have advanced identification equipment, is to immerse the stone in a flat container of methylene iodide and re-examine it with a loupe. If under these circumstances inclusions are still not visible, it cannot be identified.

Suppose you are requested by a customer to identify a piece of mottled green-and-white rough material. What would you do? Particularly in the West, jewelers often are called on by prospectors and rock hounds to make an identification of this kind. Experts recommend answering these requests by saying you are not equipped to identify uncut gem materials. Actually, you are equipped to take only one or two courses of action : test its hardness against glass or steel, or break off a small piece and make an S.G. determination. Fine-quality jadeite either barely floats or sinks very slowly in methylene iodide. Nephrite usually sinks slowly in bromoform , but it may barely float. If you wish to accommodate a customer with such a request, these two tests could be performed, but the effort is not likely to be worth while. The best solution is to suggest having a powder analysis made at a mineralogical laboratory. Most major universities have the necessary equipment to perform this test.

Suppose you have a mounted colorless stone to identify. Although it seems brilliant and shows a considerable amount of fire, you question whether it is diamond. Careful examination under a loupe through the crown facets discloses strong doubling. When placed on the refractometer the shadow edge extends all the way to the liquid at 1.81. What is the stone ? The common possibilities are zircon and synthetic rutile. The latter always has a distinct yellowish cast, and its low hardness usually is evident in the form of rounded facet edges. In addition, the dispersion is so great that the stone is almost opalescent in appearance. Zircon is either colorless or pale blue and usually is well polished. It has natural inclusions, whereas those in rutile, if any are present, are bubbles.

Summary

The foregoing discussion is intended to provide a general introduction to complete identifications, and also an insight into the thinking of the gemologist as he undertakes to identify an unknown gemstone. It will not be long before you will be able to identify almost any gemstone, if each new test of your training , ability and equipment is approached with logic. Almost every identification can teach you something new. Some of our most successful and best-liked acquaintances are those who believe that each person they meet has something to teach them. Likewise, the most successful gemologist is the one who learns something from every stone he sees and tests.

The Gem Identification Assignments are designed to teach three things

  1. the effective use of instruments,
  2. the correlation of the information gathered, and
  3. determination of the identity of the stone being tested.

There is much that is essential to this threefold objective that is to be found not in this course but in the Colored-Stone Assignments. The Colored Stone Assignments has six major goals :

  1. To provide an understanding of the nature of the properties that are used to identify stones,
  2. to explain the causes of the exceptional beauty of gemstones,
  3. to furnish a background in gem lore,
  4. to teach gem evaluation,
  5. to suggest methods for increasing gem sales; and
  6. to teach the means by which each gem may be distinguished from those that may resemble it.

Remember to make a point of using the Colored-Stone Assignment, a reference source for the stones you consider possibilities. Full in formation on the separation of each of the stones from others most likely to resemble it is given in the assignment on that gem.