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Most approaches
to colored gemstone grading rely on the four C’s, namely color,
cut, clarity and carat. Color is normally divided into the categories
of hue, saturation and tone. Cut addresses primarily the percentage
of brilliance in the stone viewed face up and secondarily the proportions
of a given gem, i.e. length, width and depth. And although some
systems, including the one used by The Guide, distinguish
between eye and loupe clean stones, clarity generally, at least
in colored stones, addresses eye flawlessness, or the relative lack
thereof, in a gem viewed face up. Carat weight, being merely a quantitative
measure of weight, has no place in the discussion at all.
In Philosophy a distinction is made between conditions
that are necessary and those that are sufficient. In the formation
of a human embryo, the male sperm and the female egg are both necessary
conditions. But, it takes two to tango. Both are necessary, but
neither is sufficient. Only when they come together do they create
the sufficient condition. The attempt to use the three C’s
as the sole criteria for quality grading gems, there is a similar
situation. All three—color, clarity and cut-- are necessary
conditions, but taken together, particularly when grading higher
quality gems they are not sufficient.
A Tale of Two Sapphires:
What is it that distinguishes a fine natural
blue Sri Lankan sapphire from its heated counterpart? Answer: transparency,
crystal or what gemologists call diaphanity. Few
would argue the fact that the natural stone has a limpid crystalline
quality that, to the expert eye, easily distinguishes the two stones.
A heated stone may have exactly the same hue, saturation and tone,
be perfectly clean, and be exceptionally well cut, but because heating
tends to muddy the crystal it will often be less transparent.
In fact it is not possible to verbally distinguish the two stones
without using adjectives like transparency, limpid and
crystalline, all of which are synonyms for diaphanity
or what most dealers call crystal.
A Gem of the Finest Water
The crystal criterion is neither a unique
idea nor a recent discovery. As early as the Fourth century BC,
Kautilya the ancient Indian Machiavelli, lists among the qualities
of a “good gem… (That it be) “transparent and
reflecting light from inside”1.
Sometime around 1433 The Chinese Admiral Ying-yai Sheng-lan uses
the term water to describe a particularly “clean, clear”
quality of Amber2. The famous Seventeenth
century gem merchant; Jean Baptist Tavernier, used the phrase “gem
of the finest Water” to describe the finest diamonds and pearls
he saw on his six voyages to India. In the sixth edition of the
Dictionary of Gems & Gemology published by GIA, Shipley
defines water as a term occasionally used “…as
a comparative quality designation for color and transparency
of diamonds, rubies and other stones…” Color and transparency
together equal “water”. Shipley goes on to mention a
hierarchy, first water, second water, etc.,so
clearly the quality of diaphanity was recognized as an
important and indispensable criterion in quality grading from early
times.
Although water is a rather poetic term,
not easily quantifiable, I sometimes think that with our modern
mania to reduce all terms to the scientific we have lost much in
the way of romance. Another more modern synonym for diaphanity
is used to describe diamonds. The term is super-d. This
designation, one that sounds about as romantic as a brand of motor
oil, refers to antique diamonds from India’s legendary Golconda
mines – stones that are reputed to be colorless with extraordinary
transparency.
Do ultra-transparent Indian diamonds really exist?
Benjamin Zucker believes so, “Place a Golconda diamond”,
he suggests, “along side a modern, recently cut D-colour diamond
and the purity of the Golconda stone will become evident.3”
Mary Murphy Hammid in a essay on Golconda diamonds written for Christie’s
maintains: “I’ve seen the incredible transparency
that people say is characteristic of Golconda diamonds in stones
the GIA graded G and H4,”. And
why not? crystal and color are two distinct qualities.
Super-d is an obvious misnomer. These diamonds are not
ultra-white, they are ultra-transparent. Colored diamond expert
Stephen Hofer speculates that the quality of ultra transparency
may be the result of a quiescent geological environment on the Deccan
Plateau that allowed for the development of a particularly well-formed
diamond crystal lattice5.
One might wish to argue that diaphanity
is nothing more than a subset of clarity. In other words, that it
is really extremely tiny inclusions that cause a stone to exhibit
poor crystal. Perhaps the most famous example are the famous
sapphires of Kashmir the finest of which exhibit a sleepy
or fuzzy appearance resulting from light refracting through
and reflecting from myriad tiny sub-microscopic inclusions. Some
Sri Lankan sapphire as well as Tanzanian pink spinel will exhibit
similar phenomena. In these examples it is clear that inclusions
are the culprit, but diaphanity is really a distinct quality.
When we say a stone is limpid, clear or crystalline, we
are talking about a quality quite distinct and different from clarity.
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