Geographic Origin Determination of Paraiba Tourmaline

Abstract

Vivid blue to green copper-bearing tourmalines, known as Paraíba tourmalines, are recovered from deposits in Brazil, Nigeria, and Mozambique. These tourmalines are sought after for their intense colors. Prices are based, in part, on the geographic origin of a stone, and determining provenance is thus an important aspect for Paraíba tourmaline. However, their geographic origin cannot be established by standard gemological testing and/or qualitative chemical analyses. GIA has established sophisticated criteria requiring quantitative chemical analyses to determine geographic origin for these tourmalines. These criteria were based on several hundred samples from known sources spanning the three countries. Highly accurate and precise quantitative elemental concentrations for Cu, Zn, Ga, Sr, Sn, and Pb are acquired with laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS). These data can then be plotted as a function of elemental concentration for accurate geographic origin determination. In Brief • Geographic origin can have a significant impact on the value of Paraíba tourmaline. • Quantitative chemical analysis with LA-ICP-MS provides a robust tool for origin determination. • The trace elements Cu, Zn, Ga, Sr, Sn, and Pb are the most useful discriminators for Paraíba tourmaline origin. • In limited cases where there are no matching reference data, an "inconclusive" origin determination would be required.

Identification of Some Gem Quality Blue to Green Li-Tourmalines

“…Vis-NIR spectroscopy is a useful tool to identify Paraíba-type tourmalines [2,10,15,16]. In fact, Cu-bearing tourmalines are characterized by two strong absorption bands around 700 nm and 900 nm, with the second one predominant over the first due to Cu 2+ , while Fe-rich tourmalines have only an intense band at 720 nm due to Fe 2+ , sometimes along with a large band at around 520 nm and a couple of sharp bands at around 415 nm due to Mn 3+ and Mn 2+ , respectively [12,13].…”

“…Paraíba-type tourmaline was named after the Brazilian state where it was first discovered at the end of the 1980s [3][4][5], even though other deposits were later found in other Brazilian regions, as well as in Nigeria and Mozambique [6,7]. Paraíba-type tourmalines consist primarily of elbaite (or fluor-elbaite, but simply mentioned as elbaite in the text), and secondly of liddicoatite (currently known as fluor-liddicoatite but simply mentioned as liddicoatite in the text); those from Mozambique have been included in this latter group [8][9][10][11]. No Paraíba-type tourmalines from rossmanite species are known to date [2].…”

Raman and Photoluminescence Mapping of Gem Materials

“…In gemology, LA-ICP-MS has been used to analyze both major and trace elements of gem quality tourmaline for geographic origin determination [49,50] and species classification [51]. The localized analytical technique produces a representative composition of the gemstone, and thus, species identification, if the sample is chemically homogeneous.…”

Analytical Proof of Origin for Raw Materials

“…The origin of gem-quality tourmaline can be determined by trace element chemistry analyzed by LA-ICP-MS [61,62]. Binary and ternary diagrams are used to separate stones from Nigeria, Brazil, and Mozambique.…”