Michael D. Cowing FGA
AGA Certified Gem Laboratory
Abstract: Decades after the establishment in the mid-twentieth century of ultraviolet-free illumination for colour grading a diamond, an examination of diamond trade and laboratory grading practices finds nearly everyone employing some type of fluorescent tube lighting containing significant ultraviolet. This paper demonstrates and quantifies the over-grading of blue-fluorescent diamonds that often results. Simple methods are proposed that, by themselves or in combination, provide inexpensive and workable solutions to resolve this problem.
Continue reading “The Overgrading of Blue-Fluorescent Diamonds”
Nineteen years ago GIA published the results of a human experiment in the observation of diamonds with various amounts of blue fluorescence. An introductory editorial indicated that this study “should bring into question the trade’s lower “bid” prices for moderate to highly fluorescent diamonds in the better colors”.
GIA was addressing the negative publicity concerning blue fluorescent diamonds, which began during the diamond investment craze of the late 70’s, early 80’s. Since then blue fluorescence has been an obstacle to marketing, leading to discounting compared to non-fluorescent diamonds of the same color grade.
Figure 1. 25-Diamond Data Base photographed in typical artificial lighting
2. 25-Diamond Data Base in Long Wave Ultraviolet “black-light” illumination. There are 5 rows of five diamonds with the five fluorescent strengths of None, Faint, Medium, Strong, and Very Strong blue fluorescence
There are several reasons for the concern and distrust by consumers and the trade of these gemstones that glow blue when excited in the dark by ultra violet (UV) radiation, Figure 2. The reasons are mostly due to misinformation and misguided publicity except for one valid concern. That is the overgrading of color that according to members of the diamond trade is too often observed. Overgrading results from the use of UV-containing, fluorescent lighting in color grading. …..Read full article here.
Figure 2. Complete set of GIA graded diamond masters from E through M.
Over the last 20 years and despite recommendations to the contrary,
many gemmologists and appraisers have gravitated to the use of cubic
zirconia (CZ) master stone sets to assist in the colour grading of diamonds.
This investigation revisits with new insight, diamond grading technique and
methodology. It addresses the judicious use of CZ master stone sets to augment diamond masters that are smaller in size and number. Study results support the use of accurately graded, carat-size CZs in reducing the subjectivity of colour grading when only incomplete (every other grade) diamond master sets of small (under 0.4ct ) sizes are available.Figure 4. CZ Master Sets, colors E-L from Ellin’s Diamonds (top row) and GemOro (bottom row)