ISOLATING CHROMOPHORES IN PYRALSPITE GARNET – QUANTITATIVE DEFINITION OF STRENGTH OF CHROMOPHORES AND THEIR IMPACT ON COLOR CHANGE

Garnets come in a wider range of colors than any other gemstone. For most garnets multiple color-producing elements (chromophores) must be considered. In this study we selectively isolate the UV-Vis-NIR spectra of four major chromophores (Fe²⁺, Mn²⁺, Cr³⁺ and V³⁺) in pyrope-almandine-spessartine (pyralspite) garnets. This was done by making various combinations of the spectra of four representative pyralspite garnets. In order to study the color strength of these isolated chromophores and their impact on color change in garnet, we propose a new quantitative definition of chromophore strength in an isometric mineral. In this work, we suggest that chromophore strength describes the quantitative color (in L*a*b* color space) produced by a certain chromophore concentration for a specified path length. In addition, the type of illuminant used is important in the determination of chromophore strength as well [e.g. daylight-equivalent (D65) vs. incandescent illuminant (A)]. Using these isolated chromophore spectra, we can accurately reproduce hypothetical garnet UV-Vis-NIR absorption spectra with various chemical compositions. As an example of the use of this model we have quantitatively explored the impact of varying chromophore composition for color change pyralspite garnets in which we can confidently reproduce the natural variation observed in natural materials.