Photo Credit (CC 2.0 license): Matthew Hurst
Diamond quality is judged according to four criteria known as the 4 Cs - color, clarity, cut and carat weight. Colored gemstones are assessed in a very similar way to diamonds, but only using 3 Cs - color, clarity and cut. However, there is a little known characteristic that sets the very finest gems apart from more pedestrian stones - diaphaneity.
Diaphaneity is difficult to succinctly explain, but is perhaps most analogous to the transparency of a stone. The very best gemstones, those that exert a magnetic pull on observers, often possess an uncommon ultra-transparency or super-transparency that allows light to travel through the gem completely unimpeded.
Diaphaneity should not be confused with the color or clarity of a gemstone. It is possible for a colored gem to have a mediocre, washed out color, but still possess an almost inexplicably alluring sparkle driven by its exceptional transparency. Of course, the most desirable gemstones will combine excellent color with outstanding diaphaneity.
Likewise, a gem can have inclusions or flaws while still retaining diaphaneity. Perhaps the best example of this phenomenon is the almost mythical blue sapphires originating from Kashmir in Northern India. These deep, velvety blue stones are characterized by silk - tiny rutile inclusions that are only visible under high magnification. This silk grants Kashmir sapphires a soft, almost dream-like blue color. However, the presence of silk does not interfere with the diaphaneity of most Kashmir sapphires.
Diaphaneity is absolutely distinct from both clarity and color. And it gives gems a presence or depth that is singularly attractive. There is speculation among gemologists that the super-transparency of diaphaneity occurs when a gem grows unusually slowly, allowing for the creation of an unusually regular crystal lattice. This results in a gem with almost no distortions on the molecular level.
Diaphaneity has had many different names throughout the long history of the gem trade, including transparency and crystal. However, the oldest name for diaphaneity is "water". The acclaimed 17th century gem dealer to European royalty, Jean-Baptiste Tavernier, mentioned gems "of the finest water" in his writings. However, the term water was used most notably in the diamond trade of the 19th and early 20th century. In this period, before diamond grading was rigorously standardized, diamonds of the best water were very highly prized.
In a book titled "Gems and Jewels: A Connoisseur's Guide" the gem trader Benjamin Zucker underscored the importance of diaphaneity in diamonds with this quote:
"Place a Golconda diamond alongside a modern, recently cut D-colour diamond and the purity of the Golconda stone will become evident."
Located in India, the legendary Golconda diamond mines were the source of many of the world's most famous diamonds, including the Hope Diamond, the Sancy and the Regent. Indeed, almost the entire world's commercial production of diamonds before the mid 18th century originated from the Golconda mines.
Gems and Jewels: A Connoisseur's Guide (book)
The concept of diaphaneity applies to colored stones just as readily as diamonds, though. Allow me to tell you about my own experience with diaphaneity. Years ago I went on a gem-hunting expedition to the Jewelers Exchange Building in downtown Boston. There was one little hole-in-the-wall gem shop that I frequented there. On this particular day I entered and started looking through the dealer's inventory, but nothing really appealed to me.
And then something in the corner of the display case caught my eye - a magnificently sparkling stone that simply called out to me. It was a huge, 4.05 carat, vivid pumpkin orange mandarin garnet from Nigeria. Now, orange is not normally a color that excites me, but this stone was truly exceptional.
Few people know that garnets aren't just red, but also come in all colors of the rainbow. The orange Spessartite variety, widely known as mandarin garnet in the trade, is one of the most coveted. As an added bonus, garnets are one of the few types of gemstones that are not enhanced via heat, dyes, irradiation, fracture-filling or other methods. That makes these completely natural stones perfect for engagement rings or other high end jewelry.
I calmly asked the dealer his price for the orange treasure. "$125 a carat" he responded gruffly. I quickly did the math in my head - a bit over $500. I slipped my wallet out of my pocket and slowly counted $506 in cash onto the counter. "I will take it" I said quietly, trying to suppress the quiver of excitement in my voice. The dealer agreed and the deal was consummated.
Now the interesting thing about this story is what made this particular mandarin garnet so good. There were other orange garnets on display right beside the stone I chose. They were very similar in terms of hue, clarity and size. In fact, it was probable that all of these mandarin garnets were originally purchased from the same lot. It is even possible that the material they were cut from originated from the same deposit. But only one of the stones was special - the one I purchased that displayed superb diaphaneity.
Diaphaneity is a subtle characteristic. It is also exceedingly uncommon. The number of stones I have seen with truly good crystal probably amounts to perhaps one or two hundred out of many thousands (or even tens of thousands). The layperson, particularly one that only frequents chain jewelry stores, will likely never come across a stone that exhibits good, much less great diaphaneity. But, if you are a serious gemstone or jewelry connoisseur, collector or investor, the crystal of a gem matters.
Secrets of the Gem Trade (book)
If you want to learn more about diaphaneity, or gemstones in general, I highly recommend a book titled "Secrets of the Gem Trade" by gemologist Richard W. Wise. It is packed full of useful information, interesting anecdotes and beautiful color photos. For years I had been striving to find gems with excellent crystal without knowing exactly what that ephemeral quality was. But once I read Richard Wise's book, all the pieces of the puzzle finally fell into place.