goldsmiths • jewelers • gemologists



Gemstone Connoisseurship;
The Finer Points, Part I /
Crystal the Fourth C


by Richard W. Wise, G.G.

 

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.

NEXT mid.gifnext.gif