The key to the identification of the various types of gem products described in this assignment is an awareness of the fact that any unknown stone may have been altered in some way or may be an assembled stone1of some kind. In other words, the tester must be alert and be suspicious of every stone offered for identification. If it has not been backed by foil or made by cementing two or more parts together, it may have been "oiled" to conceal fractures, coated on the back, or otherwise treated in an effort to deceive.
Usually, stones in these categories can be detected by simple inspection, if they are loose or mounted so that the girdle and the pavilion are exposed to direct examination. It is difficult under either condition to conceal foil or the plane along which the halve s of a doublet are joined. Even if it is not obvious that a stone is composed of two parts, there are several tests requiring to instruments that enable a gemologist to detect it. The common garnet-and-glass doublet should be identified without difficulty by comparing the high luster of the garnet on the table with that of the glass low on the crown. In the rare event that the garnet extends to the girdle or beyond, the difference in appearance is slightly less obvious, but still not difficult to detect. With a little practice, a gemologist should become adept at separating all of the garnet-and-glass doublets from a parcel of hundreds of stones in a few minutes by this method.
The most positive method of separating a doublet or triplet when there is a distinct difference in color or R.I. between the two portions, or when there is color in the cement is by immersing it in a liquid; water is usually adequate for this purpose. (Note: Stones suspected of being joined with colored cement should never be immersed in liquids such as bromoform or methylene iodide, for these chemicals may attack the cement and affect the appearance of the stone. The stone is held in the water with tweezers grasping the girdle and viewed parallel to the plane of the girdle. A doublet usually shows a distinct difference between halves (Figure 10), and a triplet shows a strong zone of color between nearly colorless portions (Figure 11).
Garnet-and-glass doublets made in any color other than red can be
detected by the presence of a distinct red ring near the girdle when
the stone is examined down on a white surface (Figure 12). However,
since any red stone appears red around the girdle, this test is useful
only with stones of other colors.
Doublets that are not obvious to the unaided eye are usually disclosed
under magnification. Whether the parts are fused or cemented together,
the joining plane is usually apparent by the presence of bubbles or
by otherwise showing up as a flat plane. Also, there is often a difference
in transparency, inclusions or other characteristics between portions;
for example, gas bubbles and angular inclusions in the same stone are
apt to be present only in a combination of natural and artificial materials,
and most likely in garnet-and-glass doublets (Figure 13).
Although magnification is exceedingly useful in the detection of
doublets, it is necessary to be able to distinguish between planes of
inclusions or flat color zones in natural stones and the plane at which
the two parts of a doublet are in contact. Bubbles are usually present
in the cement or in the glass near the point of contact. Even if no
bubbles are evident, the change of appearance between parts, or between
the stone and the cement, should be visible under magnification.
If the two parts of an un-mounted doublet are composed of different
materials, the difference in refractive indices will be detectable on
the refractometer. This test is very rarely needed, for visual-inspection
is usually sufficient. Of course, a limiting factor is that the R.I.
of the pavilion of a mounted stone cannot be obtained.
Another, sometimes misleading, test is the use of the polariscope to detect a difference in optic character between the two parts. If one of the parts is glass, the whole will seem to have the character of the other portion, for the singly refractive part will not affect the nature of the polarization imparted by the doubly refracting part. The result is seen when the stone is examined through both parts. A stone that is immersed in a liquid (of approximately the same index as the portion of the doublet, or between the two in index and examined parallel to the plane of separation is likely to show extinction at different positions for each part. If one part is singly refractive and the other doubly refractive, this will show up clearly under these conditions. The use of the polariscope for the detection of doublets is not recommended. Except when no simpler method will suffice.
A helpful means of determining double refraction of either or both parts of a doublet or triplet is to note whether bubbles or other irregularities in the joining plane appear doubled when observed first through the crown and then through the pavilion. It is a negative test, of course, for if one is not certain that doubling is seen, no conclusion can be reached.
Unmounted opal doublets disclose the line at which the parts are cemented by simple examination with the unaided eye. The only precaution here is that precious opal often occurs in thin seams overlying common opal or "ironstone" matrix; therefore, a common opal back or a fine opal top does not always indicate a doubled. However, the line of demarcation between the portion of a solid opal doublet with play of color and that without play of color is usually quite irregular. On the other hand, the cement in an opal doublet is usually obvious, and the joining plane is thin and sharp. A bezel-set opal doublet may be unidentifiable, if common opal has been used on the back; however, a very strong light passed through the stone may allow bubbles or other separations in the cement layer to be seen. If this fails to reveal the true nature of the stone, the only alternative is to unmount it.
The identification of the other gem products discussed in this assignment usually can be effected by magnification, if the stones are unmounted. The experienced tester is always suspicious of a closed back mounting, for tit often conceals something. For instance, pale emeralds that have been coated on the back to improve color and set in closed settings may appear very authentic. The use of magnification and an overhead light source, however, will usually disclose the deception; if not; ultraviolet may reveal an unnatural fluorescent color, such as yellow. Lastly, if the stones have a fine color, there should be distinct dichroism. If repeated trials in many directions reveal only slight: dichroism or complete lack of it, coating is indicated.
An engraved back on a synthetic corundum imitation of star cap-sapphire can be detected easily, because the metal foil prevents light from passing through what appears to be a translucent stone. With magnification and observation from one side, the three directions of lines are usually visible. Also, if a colored cabochon has been used for the top, the star assumes the color of the material, rather than being white, bluish or yellowish, as in natural stones. This is not infallible. However, since colored rays are known in natural star stones.
