Saturday, July 07, 2007

Gemmology On A Shoestring

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

It is a worthwhile exercise to make a list of all those gem materials which you believe you can confidently identify by lens inspection only—provided the specimens are clean, not too small, and that the lighting in good. The following list of ‘recognizable’ gems would probably be agreed to by most experienced gemologists, and could, I am sure, be extended: diamond, zircon, demantoid, peridot, opal, amethyst, star sapphire and ruby, chrysoberyl cat’s eye, quartz cat’s eye, iolite, tourmaline, hematite, marcasite, lapis lazuli, ‘Swiss’ lapis, aventurine quartz, bloodstone, ivory, pearl, pink pearl, cultured pearl, imitation pearl, paste, ‘goldstone’, doublets, synthetic star stones, synthetic rutile, strontium titanate, blue sinter spinel. Such a list of thirty materials at least makes an encouraging start; I shall suggest later the basis on which some of the above determinations may rest, and a few simple accessories which may make these and further identifications more simple and more sure.

That the stone examined should be clean (and here I am not referring to inner cleanliness) was stipulated just now, and it is certainly well worthwhile before examining a stone or stones to do a thorough job of cleaning. This is usually quite a simple matter in water containing a little liquid detergent, using soft toothbrush to get into any nooks and crannies of the setting if the stones are mounted. If the stones are then rinsed and shaken, and placed on blotting paper close below the bulb of a desk lamp, they will soon dry. Loose stones, if large, can be handled most safely in the fingers. With small stones, tongs are necessary: these should not be too sharp-nosed, and should have a mild spring. Those who have difficulty in maintaining the correct gentle pressure on the tongs to grip the stone safely while it is being examined may find tongs fitted with a ‘slide’ helpful, as these maintain a fixed pressure. An adjustable desk lamp with an opaque shade is a virtual necessity, enabling a strong light to shine on the specimen without dazzling the eye of the observer. In using a lens, a light interlocking of the left hand holding the specimen and the right hand holding the lens is essential to maintain steadiness and constant focus.

Let us now consider some of the main attributes which enable gemstones to be recognized for what they are.

This is unquestionably the greatest aid of all, though of course it can sometimes be misleading. An attempt to sort out a parcel of mixed colorless stones will soon convince the skeptic how helpful color can be. The gemologist must also be on the look out for parti-coloration, as in many tourmalines; zoned or patchy color as in amethyst, ‘burnt amethyst’, sapphire; dichroism or change of color with direction as in ruby, tourmaline, andalusite, iolite, aquamarine.

This depends, of course upon the refractive index, but also upon the perfection of polish, which in turn depends largely upon hardness. The polished surface of a diamond will reflect 17% of a ray of light at perpendicular incidence, whereas quartz under the same conditions reflects only 4½% of light. Stones of intermediate refractive index, of course, reflect to extents between these two extremes, in accordance with Fresnel’s well-known formula. The luster of a diamond is certainly one of its outstanding characteristics, and luster can often play a part in distinguishing between similar gems, e.g. between chrysoberyl cat’s eye and quartz cat’s eye.

The gemologist will know that the effect known as ‘fire’ depends upon the ‘dispersion’ of a stone, which can be stated numerically as the difference between its refractive index for red light of chosen wavelength and for a chosen wavelength of violet light. Fire is most necessary in a colorless stone, if it is to have any beauty and liveliness. Diamond, of course, is our standard here, though considering its high index of refraction its dispersion is decidedly low. By comparison synthetic rutile and even strontium titanate seem to show a rather gaudy display of flashes of spectrum colors. Demantoid garnet owes its lively appearance both to its high luster and its fire: these features should serve to distinguish it at once from either emerald or peridot, even if its color did not. Although the dispersion of synthetic white spinel is only a little higher than that of white sapphire, it does show perceptibly more fire and this makes it a more plausible substitute for diamond, particularly in step cut form.

This is contributory factor in the appearance of gemstones which is insufficiently appreciated. Most of the stones used in jewelry would be listed as ‘transparent’, and this would be true insofar as one could read print through a polished block of the stones concerned. But perfect transparency is possessed by very few minerals—diamond, synthetic spinel, and white topaz amongst them—while others, such as zircon, are almost always marred by a slight touch of milkiness. Perfect transparency is, of course, more important in colorless than in colored stones.

Double Refraction
The detection of double refraction in transparent gemstones, and the approximate assessment of its strength, are matters of prime importance in the lens identification of a given specimen, and it is here that a gemologist should score heavily over his unscientific colleagues. The ‘doubling of the back facets’ when viewed through the front of a stone with a lens is very easily in zircon (double refraction 0.06) and sphene (0.13), also in peridot (0.036) and even tourmaline (0.02): but one may need considerable skill in detecting it in quartz and in topaz (0.01, approx.) unless the stone be a large one. It is very important to remember that in all doubly refracting stones there are either one or two ‘optic axes’ along which no double refraction can be observed, and that at right angles to these directions the doubling cannot be seen either, since one image is directly behind the other. Thus one must turn and twist the stone, peering through it at the further facet junctions in all possible directions before deciding whether or not D.R is present, and if so, approximately how strong. Naturally, the larger the stone the greater the effect, and this must be taken into consideration in any assessment made.

Gemmology On A Shoestring (continued)

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