Exceptional set of tourmalines

By Dr. M.S. Krzemnicki, first published in Facette 29 (May 2024)

Figure 1: A layout of six Paraiba tourmalines of exceptional quality and size (from 4.75 to 32.62 ct) were recently submitted to SSEF for testing. Photo: L. Phan, SSEF.
Paraiba tourmaline is among the most highly prized of gems today and they are sought after for their vivid colour ranging from blue to green. Although Mozambique is known to have produced occasionally copper-bearing tourmalines in large size and exceptional colour and clarity, we were quite stunned when a client submitted a set of six Paraiba tourmalines of matching vivid (‘neon’ or ‘electric’) blue colour and excellent quality. Trace element analyses quickly revealed copper as a main colouring element in all these tourmalines, and further confirmed their reported origin from the alluvial gem deposit near Mavuco in the Alto Ligonha pegmatite district in northern Mozambique. This could be best deciphered by visualising the chemical data in a three-dimensional t-SNE scatter plot (using GemTOF trace element data) in which data clusters of Paraiba tourmalines from Brazil, Mozambique and Nigeria can be distinguished (Figure 2). Such visualization of high-dimensional chemical data is based on machine learning algorithms which are applied by SSEF since several years as a supporting tool for the origin determination of gemstones. We were pleased to see that the suite of the four smaller Paraiba tourmalines (in total 31.01 ct) – assembled by Joseph Ambalu of Amba Gem Corp.- shortly afterwards won the prestigious Cutting Edge Award by the American Gem Trade Association and were on display during the AGTA Show in Tucson early February 2024.
Figure 2: Three-dimensional t-SNE scatter plot revealing that the above described copper- bearing tourmalines originate from Mozambique. Figure: M.S. Krzemnicki, SSEF.