Spectral Differentiation Between Copper and Iron Colorants in Gem Tourmalines
The authors used Vis-NIR spectral measurements combined with LA-ICP-MS data to investigate the usefulness of absorption spectra for differentiating between copper and iron as sources of greenish blue coloration in gem tourmaline. While both Cu2+ and Fe2+ produce absorption bands with maxima near 700 nm, Cu2+ also has a strong band with a maximum near 900–925 nm, where absorption due to Fe2+ is typically at a minimum. In addition, Vis-NIR spectroscopy successfully identified Cu in pink/purple and violet stones that could be candidates for heat treatment. For the blue, green, and violet Cu-bearing tourmalines of pale-to-moderate color intensity in this study, weight percent of CuO could be estimated from the absorbance at 900 nm. This relatively inexpensive identification method may prove to be a valuable screening tool for Cu-bearing tourmaline.