Color Grading Blue Fluorescent Diamonds

[michaelgem]

They have to select some standardized light source when grading diamonds since only grading them during the day or a certain part of the day is not reasonable. Selecting lighting that contains UV content simulating that of natural sunlight sounds very reasonable to me.

I''d love to hear an argument against it. Kenny

Until the late 90''s GIA and the diamond trade always taught and required the grading of diamonds for their "true body color", the color unimproved by blue fluorescence. This is the color seen at night or out of natural daylight at normal viewing distances from most all forms of overhead illumination including fluorescent lights. Grading for the unenhanced true color was taught and believed to be accomplished by grading in the GIA Lab standard DiamondLite, which was said to have a "minimum of UV" or by grading in lighting filtered to remove the UV.

Both LWUV (Long Wave Ultra Violet) between 330nm and 390nm and VV (Visible Violet) from 390nm to 415nm excite blue fluorescence in many "Cape Series" or Type 1a diamonds, which comprise over 98% of gem quality diamonds. But at normal viewing distances from overhead lighting (3 to 4 feet or more) there is not enough of either to cause grade whitening fluorescence because of the rapid fall off of energy with distance from the light. So the diamond''s true color is seen at night or out of natural daylight.

Unfortunately, since the advent of fluorescent lighting in diamond grading in the 50''s most labs and the diamond bourses have been grading within seven inches of unfiltered tubes where there is significant grade whitening UV and VV causing the highly variable amounts of over grading of many blue fluorescent diamonds, especially those with Strong and Very Strong fluorescence grades.

The discovery of a "good deal" of UV (measured in excess of 150 uW/cm2 at a two inch grading distance from the tubes) in the DiamondLite in the late 90''s resulted in a rethinking and change in GIA lab color grading practice. This change was instituted in 2000 with the replacement of the DiamondLite with the DiamondDock. The new lighting standard to grade diamond color published in G&G Dec 2008 is the equivalent of the DiamondDock''s unfiltered fluorescent tubes having specifications that include:

1. Stable, fluorescent lamps 17 in. (43 cm) or longer
2. An intensity of light in the range of 2000-4500lux at the surface of the grading tray
3. An 8-10 in. Distance between the lamps and the grading tray
4. A color spectrum close to CIE D55-D65
5. An emission for long-wave UV (between 315 and 400nm, close to the reference spectrum of D55-D65)

So, GIA has gone from requiring a minimum of UV in color grading to requiring LWUV close to the CIE reference spectrum for daylight. The Fluorescence Study of over grading found that this amounts to roughly 33 uW/cm2 of fluorescence stimulating UV.

First published in the Austrialian Valuer thanks to the efforts of Garry Holloway and Hylda Bracewell was an IMO "illuminating" article dealing with this whole issue of the current over grading of many blue fluorescent diamonds. Another version just off the press from the British Gem-A''s publication, Gems & Jewellery has been uploaded with permission to my website where you can download a copy at:

http://acagemlab.com/temp/gemsjewellery.pdf

Members of the AGA Task Force on Lighting Standards, Garry, I and others hope to get the word out to the trade, jewelers, and consumers to get back to grading the diamond''s true color unenhanced by fluorescence in order to remove the mystery from the color grading of blue fluorescent diamonds and remove the resulting stigma attached to them.

Would you rather pay for the most often seen true color, or the highly variable fluorescence enhanced color only seen in daylight that is also highly variable? How much UV and VV fluorescence stimulation is that?

Let us know what you think.

Michael D Cowing


Will follow with the Fluorescence Study''s recommendations

 

[michaelgem]

Together with the AGA Task Force on Lighting Standards, I and a growing international body of gemmologists and appraisers/valuers, recommend a new procedure when examining fluorescent stones. Before grading them for their fluorescence-improved colour in standard daylight-equivalent light, examine them for their unenhanced body colour in lighting both filtered to eliminate UV and diffused to reduce light intensity, so that neither UV nor visible-violet excite blue fluorescence.

Based upon the new investigation and study, we recommend the use of a polycarbonate plastic (Lexan or Makrolan are examples) as an effective and inexpensive filter to remove UV from existing grading illumination. Additionally, a white plastic diffuser is recommended to reduce the visible light intensity below 400 foot-candles (fc). The study found fluorescent light intensities below 400 fc contain insufficient visible-violet to stimulate noticeable fluorescence. In addition to lowering the light intensity, such white diffusers were recommended by GIA to reduce spectral reflections and glare from diamonds being graded and to help filter out UV emissions.

An equally effective solution in concert with the movement to the ''green'' technology of LED lighting is the use of white LEDs such as the investigation''s Dazor LED desk lamp. It not only provides inherently UV-free grading light, but is dimmable without change in colour temperature down to 200-400 fc (2000-4000 lux) to avoid stimulating noticeable blue fluorescence.

A return to lighting standards that allow grading of a diamond''s colour unimproved by blue fluorescence would benefit the diamond industry in a variety of ways. First it would remove the distrust and stigma attached to fluorescent diamonds. Second, the rarer blue-fluorescent diamonds that hold their high-white colour in the absence of stimulation of fluorescence would be recognized once again for their superior beauty and rarity. Thirdly, diamonds graded without their blue fluorescence would be shown to whiten, and sometimes appear blue-white in natural daylight, giving them the edge in marketing over their non-fluorescent counterparts that they used to enjoy.

In the final analysis, it would be as beneficial as it is simple and affordable to correct the problem of over-grading blue-fluorescent diamonds. By so doing, we would restore the colour-grading of diamonds based on the traditional diamond industry standard. By grading a diamond''s ''true colour'' unenhanced by blue fluorescence the diamond world could once again enjoy worry-free commerce in blue fluorescent diamonds.

In short, using new lighting techniques, we urge a return to the past colour-grading practices that involved emphasis on the diamond''s colour in fluorescence-deactivated lighting to perform fair colour assessments of blue fluorescent diamonds and to ferret out the true blue-whites. By so doing, the diamond trade would be creating a wider canopy of consumer protection than presently exists.

Michael D Cowing

 

[kenny]

Mr. Cowing, thank you for starting this thread.
I will be interested in following it.