Friday, March 6, 2009

Improving Diamonds - Part I

The price of a diamond is determines by its physical properties and the rarer the stone, the more valuable it is. Most diamonds are somewhat yellow or brown in color and only about twenty per cent of rough stones from the mines yield flawless and color less diamonds. Pure white and flawless diamonds are therefore much in demand and expensive. Diamonds in deep shades of yellow, blue, green or even red are extremely rare in nature and so count as expensive collector’s items. Even rarer among these richly colored stones are the photochromic or chameleon diamonds that changes their color slowly on exposure to light. Obviously there is great profit if a diamond can be colored to a deep shade or made photochromic.

Jewelers have tried to enhance the color of gemstones for centuries by painting or dying the gem but tricks of this kind can easily be detected. As long ago as 1568AD, Benvenuto Cellini of Firenze dyed commonly available yellow diamonds with blue indigo to turn them green and sold as rarities.

The dye faded soon in time as all dyes do. In the last few years, a new process has been developed whereby a coat of colored synthetic diamond film is applied to the lower-girdle facets of a polished stone in order to simulate color in set diamonds. This is more stable than dying. The diamond film can withstand temperature of up to 600C and this is not affected even when the stones are heated to 600C during setting. However the coating will come off or be damaged when the diamond is re-cut or re-polished.

After cutting and polishing a diamond may exhibit cracks or metal inclusions that are seen as black or brown spots. Such stones are rejected, as they are supposed to bring bad luck. The trade has methods to remove these spots and hide the cracks and flaws. A highly focused laser beam is used to drill a very small hole up to the spot in the diamond. The stone is then boiled as acid to dissolve the inclusion. The tubular hole that reaches the surface is then filled with some colorless plastic or resin of high refractive index. The hole and filling are so small that a jeweler’s loupe cannot detect that the stone has been repaired. However, it is hardly worth doing this for small diamonds.

Microscopic examination of the filled stone can reveal that it has been doctored. When the treated diamond is tilted back and forth under a microscope, the reflected color sometimes changes from orange to blue and back to orange again in a flashing manner. The plastic filler also appears glassy, with an unnatural, melted look. Often air bubbles are trapped in the filler and near the surface there could be a cracked appearance.

The trouble with the plastic or resin filling is that in time; the plastic discolors, making the stone look worse than it did before the filling took place. A better but more difficult process forces lead glass of high refractive index into the hole, making a more permanent seal. Modern techniques using X-Rays can detect this lead glass filling as the filled areas appear opaque to x-rays. Another method for detecting this fracture filling is to examine the diamond by x-ray fluorescence, now commonly used to check gold purity. The lead in the glass filling signals it presence in the fluorescence detector.

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