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- Title
VANADIUM- AND CHROMIUM-BEARING PINK PYROPE GARNET: CHARACTERIZATION AND QUANTITATIVE COLORIMETRIC ANALYSIS.
- Authors
Ziyin Sun; Palke, Aaron C.; Renfro, Nathan
- Abstract
A type of pink pyrope garnet containing vanadium and chromium, believed to have been mined in Tanzania, appeared at the 2015 Tucson shows. The material shows a noticeable color difference from purplish pink under incandescent light (A) to purple under daylight-equivalent light (D65). This study reports a quantitative analysis of the difference in color between the two lighting conditions, based on the use of high-quality visible absorption spectroscopy to calculate CIELAB 1976 colorimetric coordinates. L*, a*, and b* colorimetric parameters were calculated for a wide range of path lengths as extrapolated from visible absorption spectra of thinner samples. Using this method, the path length of light through the stone that produces the optimal color difference can be calculated. This path length can then be used to determine the optimal depth range to maximize color change for a round brilliant of a specific material. The pink pyrope studied here can be designated as "color-change" garnet according to certain classification schemes proposed by other researchers. In many of these schemes, however, the material fails to exceed the minimum requirements for quantitative color difference and hue angle difference to be described as "color-change." Nonetheless, there is no simple solution to the problem of applying color coordinates to classify color-change phenomena. Also presented is a method by which spectra can be corrected for reflection loss and accurately extrapolated to stones with various path lengths.
- Subjects
GARNET; VANADIUM; CHROMIUM; COLORIMETRY; INCANDESCENT electric lighting
- Publication
Gems & Gemology, 2015, Vol 51, Issue 4, p348
- ISSN
0016-626X
- Publication type
Article
- DOI
10.5741/GEMS.51.4.348