Gemmology On A Shoestring

(via The Journal of Gemmology, Vol.10, No.3, July 1966) B W Anderson writes:

Hardness
Hardness as a test has always been considered taboo amongst gemologist, partly because of the danger it implies of damaging the specimen tested, and partly because of its inexactness compared with refractive index or density determinations. But hardness has a marked influence on the degree of polish that a stone can take and maintain and thus affects the appearance of a stone. The sharp facet edges in diamond, for instance, help one to distinguish it from strontium titanate, where the edges often have an almost molded appearance. It is as well to realize that hard stones such as sapphire or even diamond may show an astonishing degree of wear, and one should be careful to avoid jumping to conclusions on this evidence alone. Since the hardness of diamond is unique, a careful trial with an edge of a suspected diamond on a piece of synthetic corundum is sometimes justified. If the specimen ‘bites’ on the corundum and leaves a definite mark, there is nothing else but diamond that it can be. Before the test is done, the corundum surface should be examined with a lens to ensure that no scratches are already there on the part that is to be used; and any mark made by the diamond should be rubbed with the finger and examined with a lens to ensure that it really is a scratch. With jade and the jade-like minerals gentle trials with a knife blade or needle point may yield valuable information, but should only be used where other tests fail, and when one can be sure that no damage results to the specimen. One should also recognize that there may be considerable variations in the hardness of such materials.

Density
Before the coming of the jeweler’s refractometer devised by the late Dr Herbert Smith, tests for specific gravity (density) were the only accurate means of determining the nature of any unmounted gemstone. It remains a thoroughly useful test for any stone free from its setting. Every gemologist knows that a simple trial in a heavy liquid will distinguish at once between yellow quartz and true topaz, or between quartz or chrysoberyl cat’s eyes. To make these distinctions by the ‘feel’ or ‘heft’ of the stone in your hand is a decidedly tricky business, but worth practicing. Even when strung a necklace, the extreme ‘lightness’ of amber can be noticeable, when compared with bakelite or other synthetic resins. The judgment of the weight of a stone in relation to its ‘spread’ is, of course closely bound up with a knowledge of its specific gravity, and here again the gemologist scores.

Cleavage
Where a stone has a marked cleavage, traces of this can often be noticed as flaws or incipient flaws within the stone or on the surface, where any nicks or chips may be seen to have flat sides instead of curved surfaces as in a conchoidal fracture. Cleavage nicks can often be detected round the girdle of a brilliant cut diamond, especially if the stone had been removed from a setting. This adds one more to the many revealing signs that one can find when examining a diamond.

Surface structures
This is rather a comprehensive heading, and can be used to cover such things as the traces of untouched crystal surface (naturals) that can often be detected on the girdle of a cut diamond; similar structures on the rear facets (really crystal faces) of a Lechleitner synthetic emerald, where the overgrowth is purposely left to enhance the color; the ‘flame’ pattern which is so completely distinctive for pink (conch) pearl, and so different from the grained structure of coral; the ‘engine-turned’ pattern on the surface of ivory; the dimpled surface of the fine jadeite; the demarcation line (nearly always above the girdle) marked by a sharp change in luster, when the surface of a garnet-topped doublet is examined in reflected light; the short, parallel crack-like markings (fire marks) due to careless cutting, seen only in corundum, more particularly in synthetic ruby or sapphire; and so on. A complete list of surface signs would be a very long one.

Internal structures
Internal structures or ‘inclusions’ should not really have come so late in the batting order, since such features are often of enormous help in identifying different gemstones and in distinguishing natural stones from their synthetic counterparts. But to study inclusions in their full beauty and detail one undoubtedly needs to examine them under a microscope—preferably a binocular microscope, and immersed in a cell of suitable fluid. Even with a pocket lens some inclusions are completely distinctive: for instance, the ‘horsetail’ inclusions of asbestos fibres which are almost invariably to be seen in demantoid garnet, and the ‘silk’ which is so typical a feature of Burma ruby. In both these cases of course the color and appearance of the stone will already have put you on the right track. The list of such ‘deadsure’ identifications by inclusions I not a very long one, however, except at the gifted hands and eyes of a master of the subject such as Dr Edward Gubelin. One should, however, be certain of an amethyst which shows the curious ‘tiger-stripe’ inclusions; of ‘goldstone’ (aventurine glass) with its glittering triangles of included copper; of a paste when it shows a typical elliptical bubble or so and of a doublet when a layer of bubbles can be seen at the surface where garnet meets the glass. Hessonite garnet, of course, is usually easy to detect with its crowded inclusion-picture of diopside crystals in a treacly golden brown setting. The curved lines in synthetic ruby may be too finely spaced for a lens to detect, but in sapphire the broader curved swathes of color are often more visible to the lens than under the microscope. The stone should be viewed against the white background of a sheet of clean paper or blotting paper, and turned in different directions until the right angle for viewing is found. Immersion in liquid will often help here (I am jumping ahead a little) and will also reveal the straight-sided bands of color so typical of natural sapphire. The ‘feel’ of a stone and its ‘coldness to the touch’ may also on occasion provide evidence of its nature.

Gemmology On A Shoestring (continued)