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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.
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