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Figure 1. These 12 intensely colored copper-bearing tourmalines (0.31–1.04 ct) are reportedly from a new locality in Nigeria. Courtesy of Barker & Co.; photo by Robert Weldon.
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Copper-bearing (Paraíba-type) tourmaline is currently known from three countries: Brazil, Nigeria, and Mozambique. Within a given country, stones from different mines (in the case of Brazil and Nigeria) show variations in color and composition (e.g., A. Abduriyim et al., “‘Paraíba’-type copper-bearing tourmaline from Brazil, Nigeria, and Mozambique: Chemical fingerprinting by LA-ICP-MS,” Spring 2006 Gems & Gemology, pp. 4–21). However, most of the Nigerian stones we have examined at GIA were remarkably consistent in color (light blue to green) and trace-element composition (enriched in Pb).
In October 2007, Bill Barker (Barker & Co., Scottsdale, Arizona) informed these contributors about a new source of Cu-bearing tourmaline in Nigeria that has produced material with a wide range of saturated colors similar to those from the original source of Paraíba tourmaline at Mina da Batalha, Paraíba State, Brazil. These colors were quite different from those seen previously in Nigerian material. According to his supplier, the stones came from a different area than those that have been mined previously for this tourmaline. All of the rough material consisted of broken fragments, so it was not possible to determine whether they originated from a primary or secondary deposit. So far Mr. Barker has obtained approximately 100 g of rough, from which 24 stones have been faceted in weights ranging from 0.2 to 4 ct.
Mr. Barker loaned 12 of the faceted stones (0.31–1.04 ct; figure 1) to GIA for examination. They showed highly saturated violet to bluish violet (five stones), blue (two), bluish green (one), green (two), and purple (two) colors that we had not previously seen in Nigerian Cu-bearing tourmaline. Only the two blue stones and one of the bluish green samples were reported to have been possibly heat treated. The following properties were obtained on the 12 samples: RI—1.620–1.643; SG—3.01–3.12; and inert to both long- and short-wave UV radiation. Microscopic examination revealed typical “trichites,” growth tubes, and two-phase (liquid and gas) inclusions in nearly all samples.
EDXRF chemical analysis confirmed that the tourmalines contained significant concentrations of Cu, and LA-ICP-MS indicated Cu contents up to ~1.5 wt.%. Interestingly, most of these intensely colored tourmalines had relatively low Pb concentrations (<40 ppm). These new Nigerian stones show a strong resemblance to their Brazilian counterparts in both color and chemistry. They provide a good reminder of the difficulties associated with determining the country of origin for gems, given the almost inevitable discovery of new localities. Thorough characterization of gems as they are produced from new mines allows gemological laboratories to remain confident in their ability to determine their geographic origin.
Christopher M. Breeding and Kimberly Rockwell
GIA Laboratory, Carlsbad
Brendan M. Laurs
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