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How to facet gem-quality chrysoberyl: Clues from the relationship between color and pleochroism, with spectroscopic analysis and colorimetric parameters

  • Ziyin Sun , Aaron C. Palke EMAIL logo , Jonathan Muyal and Robison McMurtry
Published/Copyright: July 31, 2017
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American Mineralogist
From the journal American Mineralogist Volume 102 Issue 8

Abstract

Pleochroism plays an important role in determining the face-up visual color appearance of faceted, optically anisotropic (non-cubic) gemstones. One area that has received little attention is the interplay between pleochroism and the so-called alexandrite effect wherein the perceived color of a mineral changes with different lighting conditions (i.e., daylight vs. incandescent light). In this article we have collected ultraviolet/visible/near-infrared (UV-Vis-NIR) spectra of a gem-quality, synthetic Cr-bearing chrysoberyl crystal along its three crystallographic axes. We use these spectra to calculate the color and to quantify the color change that would be observed in a wafer or faceted gemstone in any orientation and for any prescribed path length of light between 1 and 25 mm. We describe the method used to perform these calculations and give an overview of color science and color space as it pertains to mineralogy and gemology. The data collected here are used to predict the optimum orientation for a wafer or a faceted alexandrite gemstone to produce the maximum color change sensation between daylight and an incandescent light source. We find that a wafer oriented with the unpolarized light-path-length perpendicular to the a-axis exhibits the strongest color change but that the color change is weaker parallel to the a-axis. Pleochroism in a faceted stone will mix light traveling in different directions. This relaxes requirements to orient a stone along the “best” direction, but it is still found that stones cut with their table to culet direction oriented perpendicular to the a-axis show the best color-change while orientation parallel to the a-axis produces weaker color change. Nonetheless, there is a wide range of “acceptable” orientations and no single “best” direction for a faceted gemstone. The results of this study demonstrate the complex nature of color in minerals and shed light on the intricate interplay between several factors including pleochroism, lighting conditions, light path length through a transparent sample, and chromophore concentrations. The use of the techniques outlined here can lead to a better understanding of the color sciences in the mineral world in general.

Keywords: Alexandrite; alexandrite effect; pleochroism; Usambara effect; visible spectroscopy; colorimetry

Acknowledgments

The authors thank Elise Skalwold for her thorough review as well as the associate editor Aaron Celestian for handling the manuscript. Many thanks are also owed to Chi Ma of Caltech for his assistance with EPMA measurements. The authors thank James Shigley from Gemological Institute of America for his constructive comments. We thank Mike Breeding, Dino DeGhionno, Shane McClure, Nathan Renfro, David Nelson, Troy Ardon, and Tao Hsu from Gemological Institute of America and David Patterson from the Geminex Corporation for many helpful mineralogical and colorimetric discussions. This study was supported by Gem Identification Department in Gemological Institute of America, Carlsbad, U.S.A.

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Received: 2016-11-11
Accepted: 2017-4-10
Published Online: 2017-7-31
Published in Print: 2017-8-28

© 2017 by Walter de Gruyter Berlin/Boston

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https://doi.org/10.2138/am-2017-6011
Keywords for this article
Alexandrite; alexandrite effect; pleochroism; Usambara effect; visible spectroscopy; colorimetry

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  2. How many boron minerals occur in Earth’s upper crust?
  3. Outlooks in Earth and Planetary Materials
  4. Network analysis of mineralogical systems
  5. Special collection: From magmas to ore deposits
  6. Geochemistry of the Cretaceous Kaskanak Batholith and genesis of the Pebble porphyry Cu-Au-Mo deposit, Southwest Alaska
  7. Special collection: From magmas to ore deposits
  8. Physicochemical controls on bismuth mineralization: An example from Moutoulas, Serifos Island, Cyclades, Greece
  9. Special collection: Earth analogs for martian geological materials and processes
  10. Geochemistry and mineralogy of a saprolite developed on Columbia River Basalt: Secondary clay formation, element leaching, and mass balance during weathering
  11. Special collection: Apatite: A common mineral, uncommonly versatile
  12. An ab-initio study of the energetics and geometry of sulfide, sulfite, and sulfate incorporation into apatite: The thermodynamic basis for using this system as an oxybarometer
  13. Special collection: Dynamics of magmatic processes
  14. The role of modifier cations in network cation coordination increases with pressure in aluminosilicate glasses and melts from 1 to 3 GPa
  16. Nitrides and carbonitrides from the lowermost mantle and their importance in the search for Earth’s “lost” nitrogen
  18. Accounting for the species-dependence of the 3500 cm−1 H2Ot infrared molar absorptivity coefficient: Implications for hydrated volcanic glasses
  20. A finite strain approach to thermal expansivity’s pressure dependence
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  24. Single-crystal equations of state of magnesiowüstite at high pressures
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