Lab Notes Gems & Gemology, Fall 2022, Vol. 58, No. 3

Cat’s-Eye Paraíba Tourmaline with Copper Inclusions

The copper inclusions produce a golden chatoyancy.
Figure 1. This 1.33 ct cat’s-eye Paraíba tourmaline with copper inclusions shows a golden chatoyancy. Photo by Shunsuke Nagai.

The Tokyo laboratory recently received a 1.33 ct greenish blue oval cabochon measuring 6.83 × 6.17 × 3.55 mm (figure 1) and displaying a cat’s-eye effect. Standard gemological testing yielded a refractive index of 1.620–1.640, a uniaxial optical sign, and a hydrostatic specific gravity of 3.08, all consistent with tourmaline. Ultraviolet/visible/near-infrared spectrometry showed the absorption band at 900 nm, which is greater than the one at 700 nm, indicating the greenish blue color of this sample was dominated by copper (P.B. Merkel and C.M. Breeding, “Spectral differentiation between copper and iron colorants in gem tourmaline,” Summer 2009 G&G, pp. 112–119). This stone meets the requirement for designation as a Paraíba tourmaline (Laboratory Manual Harmonisation Committee Information Sheet #6, 2012). Laser ablation–inductively coupled plasma–mass spectrometry indicated a high copper concentration of 11741–14794 ppmw. The high copper (over 10000 ppmw) is limited to Brazilian origin (Y. Katsurada et al., “Geographic origin determination of Paraíba tourmaline,” Winter 2019 G&G, pp. 648–659).

As previously reported (Winter 2018 Lab Notes, pp. 438–439), cat’s-eye Paraíba tourmaline usually includes groups of parallel tube-like inclusions that create the phenomenon. However, that was not the case for this tourmaline.

Light is reflected by the copper inclusions.
Figure 2. The copper inclusions on the lower right are partially reflecting light. Photomicrograph by Yuxiao Li; field of view 3.5 mm.

Different from normal cat’s-eye Paraíba tourmaline, the band of reflected light in this stone was stronger, with metallic luster. Microscopic examination revealed a fluid inclusion network, rounded metallic inclusions, and growth tubes, with many golden-colored dendritic inclusions causing the cat’s-eye effect (figures 2 and 3). Raman spectroscopy could not be used to identify the inclusions, which were too thin and did not reach the surface. Such inclusions in Paraíba tourmaline have been reported and identified as natural copper (F. Brandstätter and G. Niedermayr, “Copper and tenorite inclusions in cuprian-elbaite tourmaline from Paraíba, Brazil,” Fall 1994 G&G, pp. 178–183). The natural copper inclusions were aligned in the same direction, parallel to each other, and reflected the light to cause the chatoyant effect. This also explains why the reflective light band in this stone looked different from other cat’s-eye Paraíba tourmaline.

The copper inclusions display golden luster when light enters the tourmaline from a certain direction.
Figure 3. The copper inclusions aligned in the same direction appear brownish and black (left) and together display golden luster when the light enters from a certain direction (right). Photomicrographs by Yuxiao Li; field of view 2.3 mm.

While some cat’s-eye Paraíba tourmalines have been examined in GIA laboratories, this is the first with chatoyancy caused by copper inclusions.

Yuxiao Li is a staff gemologist at GIA in Tokyo.