CU AND FE CONTRIBUTIONS TO THE COLOR OF CUPRIAN TOURMALINE: CHALLENGE FOR PARAÍBA NAME JUDGEMENT

Paraíba tourmaline, a highly valued gemstone over the past three decades, is a tourmaline of blue/green color mainly due to the presence of Cu and Mn. The main coloring agent is known to be Cu. While one of the two Cu²⁺ absorption bands with maximum in the red region (approximately 700 nm) overlaps significantly with a Fe²⁺ absorption band, the other Cu-related absorption band in the near-infrared region (approximately 915 nm) does not. This makes it possible to separate Paraíba tourmaline from blue/green Fe-colored tourmalines by Vis-NIR spectroscopy. However, green cuprian tourmalines with non-negligible Fe present a challenging problem when identifying Paraíba tourmaline as it can be difficult to determine which element is the dominant chromophore. In the gemstone market, most Fe-rich cuprian tourmalines are faceted making it difficult to account for variable absorption in different crystallographic directions. This study is aimed at applying Cu and Fe contributions in oriented wafers to faceted gemstones to judge the dominant chromophore of cuprian tourmaline.

Blue/green cuprian and non-cuprian tourmalines from Brazil and Namibia were selected from GIA’s research collection. These samples were accurately oriented and fabricated to allow both the o-ray and e-ray absorption spectra to be collected. Absorption was collected with UV-Vis-NIR spectrometer and trace elements were analyzed with LA-ICP-MS on each wafer. The absorption intensities of Cu and Fe are proportional to the concentration of the elements.

The absorption spectra of o-ray and e-ray were converted to non-polarized spectra that allow comparison to spectra of faceted gemstones. When the Cu concentration of blue/green cuprian tourmaline is known, it is possible to estimate the Fe concentration required to create the same level absorption as Cu does. For example, the threshold for Fe concentration to have the same absorption as 10000 ppm Cu is approximately 25000 ppm.