Gota de Aceite–Like Effect in a Brazilian Emerald
Internal optical effects can provide useful information to help identify gemstones and understand their nature and geographic origin. For example, gota de aceite, a roiled or “drop of oil” appearance seen in Colombian emeralds, is considered to be caused by irregularities in the crystal structure due to rapid growth triggered by alteration of the growth conditions (e.g., R. Ringsrud, “Gota de aceite: Nomenclature for the finest Colombian emeralds,” Fall 2008 G&G, pp. 242–245; Winter 2017 Lab Notes, pp. 460–461).
Recently, GIA’s Tokyo laboratory examined a 2.99 ct step-cut emerald measuring 10.17 × 8.12 × 5.50 mm (figure 1) for geographic origin determination. This stone exhibited a unique roiled growth structure resembling the gota de aceite effect (figure 2, left) at an angle nearly perpendicular to the table facet. The gota de aceite structure was visible parallel to the c-axis of the emerald based on microscopic and polariscopic observations. From other directions such as girdle to girdle, the effect was not visible and columnar growth structure was observed. The emerald also contained numerous reflective raindrop-like needles and tubes (figure 2, right) and irregular two-phase inclusions. This stone had a refractive index range of 1.580–1.588 and showed no reaction under a Chelsea color filter. The ultraviolet/visible/near-infrared spectrum showed a significant Fe2+ broad band at around 810 nm. These internal and standard gemological features did not match Colombian origin, with the exception of the gota de aceite effect. Laser ablation–inductively coupled plasma–mass spectroscopy revealed a high iron concentration (4540–4800 ppmw) and medium potassium (230–254 ppmw) and lithium (62–68 ppmw) contents. A Brazilian origin was determined based on inclusions and trace element composition (S. Saeseaw et al., “Geographic origin determination of emerald,” Winter 2019 G&G, pp. 614–646).
The gota de aceite optical phenomenon has been used as a characteristic feature of classic Colombian emeralds, but a similar pattern was previously reported in a Zambian emerald (Winter 2017 Lab Notes, pp. 460–461). The similarity in the growth pattern suggests that the growth conditions causing the rapid emerald crystallization were similar, although the geological origins and chemical compositions of both emeralds are different. As several microscopic studies on emerald have concluded (e.g., S. Saeseaw et al., “Three-phase inclusions in emerald and their impact on origin determination,” Summer 2014 G&G, pp. 114–132; Winter 2017 Lab Notes, pp. 460–461), inclusions and internal features may not always be helpful in determining geographic origin, and the gota de aceite effect may not offer enough evidence for Colombian origin. Careful observation of other inclusions combined with advanced testing is required for origin determination of emerald.
