Bicolor Cuprian Tourmaline
A bicolor gem has two colors in one stone. Ametrine is a well-known bicolor gem, a purple and yellow quartz combining amethyst and citrine. In addition to the ametrine variety of quartz, other colored gemstones such as corundum and tourmaline can be naturally bicolor, although most of them do not have a specific variety name. Most bicolor gem materials are cut to show different colors that are obvious when viewed in face-up orientation.
The Tokyo laboratory recently examined a bicolor rectangular step cut weighing 1.62 ct and measuring 9.14 × 5.47 × 3.71 mm (figure 1). The color was gradually distributed from bluish green to dark yellowish green along the length. This stone was doubly refractive with a refractive index of 1.620–1.640 and a specific gravity of 3.10. Microscopic observation revealed networked fluid inclusions and strong doubling. The stone was identified as tourmaline by these gemological features.
Paraíba tourmaline, a certain type of copper-bearing tourmaline, has been one of the most sought-after gemstones in the trade over the last three decades. In 2012, the Laboratory Manual Harmonisation Committee (LMHC) updated the definition of Paraíba tourmaline as “a blue (electric blue, neon blue, violet blue), bluish green to greenish blue, green (or yellowish green) tourmaline of medium-light to high saturation and tone (relative to this variety of tourmaline), mainly due to the presence of copper and manganese.” Visible/near-infrared (Vis-NIR) absorption was collected with GIA’s custom-made UV-Vis spectrometer to determine the chromophore. To be considered Paraíba tourmaline, the copper-related absorption needs to be dominant and the color appearance should be within the defined range.
Non-polarized absorption spectra of the whole stone showed two bands in the red and near-infrared regions (figure 2, black line). To measure the spectra of each color separately, one half of the gem was covered with opaque black cardboard to allow light to pass through only one color at a time. The results indicated that the dominant chromophore was different (figure 2; bluish green and dark green lines at the middle and bottom, respectively)—the bluish green part is colored by copper, whereas the dark yellowish green part is colored by iron (P.B. Merkel and C.M. Breeding, “Spectral differentiation between copper and iron colorants in gem tourmalines,” Summer 2009 G&G, pp. 112–119).
The trace element composition of each differently colored region was analyzed by laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS). As shown in table 1, the bluish green side shows lower iron (566 ppmw) and higher copper (7850 ppmw), and the dark yellowish green side shows higher iron (11287 ppmw) and lower copper (4953 ppmw). Comparing the different Vis-NIR absorption patterns of these portions (figure 27), the association of copper and iron for each color portion is in agreement with Merkel and Breeding (2009). By virtue of its color and its chromophore, only the bluish green portion is consistent with the definition of Paraíba tourmaline.
Sometimes we encounter cuprian tourmaline with chemical zoning, which has varying trace element concentrations within. A drastic difference, as in the case of this bicolor tourmaline with a non-Paraíba iron-colored portion, is not typical. The dark yellowish green part could have been polished off to make this a typical Paraíba tourmaline.
