CHROMOPHORES OF PARAÍBA TOURMALINE IN VISIBLE RED AND NEAR INFRARED LIGHTS: CASE STUDY

Blue/green Cu-bearing tourmaline, known as Paraíba tourmaline, has been a highly valued and important gemstone for the past three decades. The Laboratory Manual Harmonisation Committee (LMHC) defines Paraíba tourmaline as a tourmaline of blue/green color mainly due to the presence of Cu and Mn. The main coloring agent is known to be Cu and the optical absorption is more intense in the E⊥c direction. While one of the two Cu²⁺ absorption bands with maximum at approximately 700 nm overlaps significantly with a Fe²⁺ absorption band, the other Cu-related absorption band at approximately 915 nm does not. This makes it possible to use UV-Vis-NIR spectroscopy to separate Paraíba tourmaline from blue/green Fe-colored tourmalines. However, there are green Cu-bearing tourmalines with non-negligible Fe. This can be challenging for Paraíba judgement as it can be uncertain which element is dominant.

Brazilian blue/green cuprian elbaite and Namibian blue/green non-cuprian elbaite were selected from the GIA’s research collection. Six of the cuprian samples and eighteen of the non-cuprian samples were accurately oriented and fabricated to allow both the o-ray and e-ray absorption spectra to be collected. UV-Vis-NIR absorption was collected and trace elements were analyzed with LA-ICP-MS on each wafer. For cuprian elbaite, absorptions at both 700 nm and 915 nm were normalized with the path lengths, and compared with Cu and Fe concentrations. For non-cuprian elbaite, absorptions at 700 nm and Fe concentrations were compared.

The results suggest that the absorption intensities of Cu and Fe are proportional to the concentration of the elements. Nearly 30000 ppm Fe is required to create the same level of absorption as that by 12000 ppm Cu at 700 nm. This study makes it possible to simulate the contribution to the color by Cu and Fe separately.

For the demand of gemstone market, it is necessary to know the thresholds of which chromophore is dominant. The study is helpful to establish the judging method for Paraíba tourmaline call.