by Richard W. Wise, G.G.
From a strict grading perspective, the inclusions that create the fuzzy appearance in some gemstones are not visible or at least not resolvable under 10x and have little effect on the clarity grade given a particular stone. In the case of Golconda or super-d diamonds, the presence or absence of ultra-transparency would have absolutely no impact on the clarity grade listed on a laboratory grading report.
Tiny inclusions are one, but only one, of the possible causes of poor crystal. Poor diaphanity has several causes. Consider a strongly blue fluorescent diamond. Such stones are often described as visibly oily due to a loss of transparency when the diamond fluoresces in daylight. Fluorescence will be noted in a grading report, but its presence or absence cannot be said to affect the stone’s clarity grade.
Many varieties of gemstones tend to lose something when viewed in certain lighting. Incandescent light is the usual culprit. Most varieties of tourmaline and garnet and some varieties of corundum seem to “close up” “muddy” or “bleed color” when exposed to the light of an ordinary light bulb. The cause is not well understood. The point is, the gem loses transparency and therefore some of its beauty. Therefore another criterion is needed to properly quality grade gemstones. That fourth “C” is crystal.
Crystal must be judged in various lighting environments. Different types of light have distinct color temperatures that are measured by units, Kelvin. North daylight at noon, the traditional gemstone grading standard, is balanced between yellow and blue at 5,500 degrees Kelvin. As Kelvin temperature decreases, light becomes yellower, and as the temperature increases the light becomes bluer. Incandescent or lightbulb light at 2,800 kelvin is distinctly yellowish. The lighting temperature determines the color of the light, and that in turn impacts the visual appearance of the gem being viewed in that light.
The tendency of gems to change appearance between natural daylight and incandescent light has traditionally been called bleeding. In blue sapphire, for example, one of the qualities that makes a Kashmir stone so desirable is that it doesn’t bleed color. Due to an absence of chromium, the color of a fine Kashmir sapphire will remain unaltered as the lighting environment is changed.
The visual appearance of other varieties of gemstones, including some ruby and sapphire and most varieties of garnet and tourmaline, will also change as the lighting environment changes. However it is not quite accurate to use the term bleeding to describe the result. I doubt if any jewelry professional who regularly works with colored gemstones has failed to notice these alterations. And although language lacks precision there is little choice but to use it to describe the visual effect, Tsavorite garnet seems to close up in incandescent light while rhodolite turns muddy and brownish. Green and blue tourmaline pick up a gray mask and appear dull and sooty like the chimney of an oil lamp. Pink to red tourmaline acquires a muddy brownish mask. Not all the effects are negative, aside from its loss of transparency, Thai ruby turns a purer red losing its purplish secondary hue when viewed in incandescent light.
The changes described affect not only crystal, but color (hue, saturation and tone) as well. Such effects are general, but not universal. For example, ninety-eight percent of all rhodolite garnet will muddy, turn brownish, losing both transparency and color saturation in incandescent light. This leaves only about two percent that retain both its color and crystal under the lightbulb. All other C’s being equal, if the stone is of high color, clean and well made, this two percent constitutes the crème de la crème of rhodolite garnet. The same may be said for pink tourmalines that do not muddy and tsavorite that retains its open color in incandescent light. Crystal becomes relatively more important in higher quality gemstones. The diaphanity of diamond with a clarity grade of I-3 is not particularly significant.
Historically crystal has also played a part in the discrimination of the finest pearls. Prior to the introduction of cultured pearls which are seeded with an opaque sphere ground from the shell of a freshwater mollusk, transparency or at least translucency was very much a characteristic valued in the finest pearls. In his Travels to India, Tavernier describes the world’s paramount pearl (circa 1670), a gem at that time in possession of a minor prince of Muscat. “This prince possesses the most beautiful pearl in the world, not by reason of its size for it only weighs 12 1/16 carats nor on account of its perfect roundness; but because it is so clear that you can almost see the light through it”6. Tavernier also repeatedly uses the term water to describe the quality of pearls.
As we have shown, diaphanity, transparency or crystal is a necessary grading criteria that deserves more than just a footnote in the discussion of quality in gemstones. Several factors including; sub-microscopic inclusions, ultraviolet fluorescence and the color of the lighting environment may impact on crystal. Thus crystal is a distinct criterion and it cannot be reduced to a subset of clarity.
The distinctions made in the course of this discussion are real in that they reflect observable phenomena that affect the beauty and desirability of gemstones. They are real also because they reflect demonstrable price differentials in the marketplace. From a grading perspective, crystal is a distinct and vitally important criterion without which it is impossible to adequately describe the finest gemstones. In short, crystal is the true fourth C of gemstone quality evaluation and connoisseurship.
1_Kautilya, The Arthashastra, Rangarajan, L.N. trans., Penguin Classics, India, 1984, pp.777-778
2_Ma Huan, Ying-yai Sheng-lan, The Overall Survey of the Ocean’s Shores, 1433, 1970 reprint, White Lotus Press, Bangkok. P.111
3_Zucker, Benjamin, Gems & Jewels, A Connoisseur’s Guide, Thames on Hudson, 1984, p.86
4_as quoted by Federman, David, Modern Jeweler’s, Gem Profile/2,Vance Publishing, 1992, p.49
5_Stephen Hofer, personal communication , October, 2000
6_Tavernier, J.B. Travels in India Vol II, 1676 edition, Ball, V., trans., Oriental Books Reprint Corporation, New Delhi, 1976 p.86
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