How GIA Color-Grades Diamonds
“What GIA’s Fluorescence Study Ignored,” by Gary Roskin (JCK, September 1998, p. 149) brought into question the use of light sources that include an ultraviolet (UV) component in the grading of diamond color in the Gemological Institute of America’s Gem Trade Laboratory. Let me explain our position further.
The grading of color in colorless to light yellow diamonds was pioneered by GIA and has been used in the examination of millions of diamonds by the GIA Gem Trade Laboratory for nearly 50 years. The procedures for color grading have been taught to thousands of our students along with the use of the DiamondLite with Verilux bulbs as a consistent lighting environment. Virtually every trade diamond-grading environment is illuminated by a source or sources with a UV component.
In the study published in the Winter 1997 issue of Gems & Gemology (“A Contribution to the Effect of Blue Fluorescence on the Appearance of Diamonds” by T.M. Moses et al., pp. 244-259), we found that the Verilux bulbs used in GIA’s diamond-grading units, standard cool-white fluorescent light bulbs, and northern hemisphere daylight (even filtered through a glass window) all have a certain amount of UV radiation. Hence the Verilux sources are similar – in terms of UV exposure – to grading environments throughout the world.
It is important to recognize that most of the environments in which consumers wear diamonds – outdoors, in daylight filtered through windows, or in fluorescent light indoors – also have UV radiation.
From our investigation, no commercially available cut-off filter blocks UV exactly at the border with visible light; these filters block some of the blue portion of the visible light, too. However, this region – from wavelengths of 400 to 500 nm – is where D to Z color diamonds show their characteristic absorptions. Our investigation of UV filters found them to have inherent colors such as green or gray.
Visible spectra of two of the filters showed that the response across the visible range was not flat, indicating that the color of the filter itself is not neutral. Inherent colors of such filters affect the color-grading results of all D to Z diamonds – fluorescent and non-fluorescent.
The availability of “UV-free” light sources is somewhat misleading in that these sources are not innately UV-free but have bulbs that have been coated with a UV-blocking filter (of unknown stability). Indeed, the actual specifications from the manufacturer cited show the UV cut-off of the coated tube to be at 388 nm, whereas the UV region extends to 400 nm. Hence the previous comments about filters are applicable. Compared with the coated Verilux (VLX) lamps, the lamps used by GIA’s Gem Trade Laboratory have a higher color temperature and color-rendering index. These illuminant qualities are even more important than the UV output of a lamp.
In addition, diamond luminescence is not limited to that induced by UV radiation. The color of diamonds is also affected by luminescence to visible light, especially that in the violet or blue range. Just as we would not recommend grading diamonds in light with no blue component, we feel it is disingenuous to do so in illumination lacking UV radiation.
Finally, the GIA Gem Trade Laboratory has a responsibility to provide accurate and consistent grading reports that are not arrived at in a rarefied environment. GIA is sensitive to centuries of diamond-grading history in a north daylight environment, in addition to the millions of grading reports issued in our laboratory over the past decades. Although we continue to look for ways to maintain our standard and improve our quality, we will not act in an irresponsible manner that might undermine those we serve – the industry and the public.
Thomas Moses, Vice President, Identification Services GIA Gem Trade Laboratory, New York
Large Diamond Incorrectly Identified
To correct gem and jewelry historian Jim Becker’s letter (“Large U.S. Diamonds,” JCK, October 1998, p. 24), the Punch Jones diamond was not found in Arkansas but near Peterstown, W.Va.
Guido Giovannini-Torelli Editor, Diamond Insight, New York, N.Y.
In the article “Fred Ward Seeks a New Day in Court” (JCK, November 1998, p. 84), the photos on page 85 should have been credited as follows: © Fred Ward, 1998.
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