Lab Notes Gems & Gemology, Winter 2017, Vol. 53, No. 4

Gota de Aceite in a Zambian Emerald

<em>Gota de Aceite</em> in an emerald from Zambia.
A muted gota de aceite effect, with epigenetic ilmenite, in an emerald from Zambia. Photo by Nathan Renfro; field of view 1.26 mm.

Gota de aceite (Spanish for “drop of oil”), a rare phenomenon that occurs in the finest emeralds, is typically associated with Colombian origin (R. Ringsrud, “Gota de aceite: Nomenclature for the finest Colombian emeralds,” Fall 2008 G&G, pp. 242–245). The phenomenon creates a roiled effect caused by irregularity in the emerald’s internal crystal structure. During the gemstone’s crystallization the growth conditions are altered, giving rise to rapid columnar growth. When looking down the emerald’s c-axis, the outline of the columns can be seen. This sometimes has the appearance of drops of oil, giving rise to the name. The columns typically have a slightly different trace element composition from that of the host emerald. This heterogeneity in trace element chemistry will cause a minor change in the refractive index of the columns, allowing the structure to be visible.

The Carlsbad laboratory recently received a 2.10 ct emerald for origin determination. The stone had blocky multiphase inclusions and ilmenite scattered throughout, along with mica platelets and a very subtle blue flash effect associated with some small feathers. At a certain angle, a muted gota de aceite effect was also visible (see above). The UV-Vis spectrum shows a distinct Fe2+ broad band at approximately 900 nm. Inductively coupled plasma–mass spectrometry (ICP-MS) confirmed the stone’s relatively high iron content and indicated a chemical composition consistent with Zambian origin.

The gota de aceite seen in this Zambian emerald was not exactly the same as the phenomenon found in Colombian emeralds. Dendritic ilmenite, an iron-rich mineral, was fully enclosed in the columnar structure. We believe the epigenetic ilmenite exsolved the increased iron content, as Zambian emeralds contain a larger amount of iron than their Colombian counterparts (J.C. Zwaan et al., “Emeralds from the Kafubu area, Zambia,” Summer 2005 G&G, pp. 116–148).

Until now, gota de aceite has only been documented in Colombian emeralds and has been an aid in origin determination. Still, this is not the first time that an inclusion indicative of a particular origin has been found elsewhere. Jagged three-phase inclusions, traditionally seen in Colombian emeralds, have also been documented in emeralds from certain mines in Afghanistan, China, and Zambia (S. Saeseaw et al., “Three-phase inclusions in emerald and their impact on origin determination,” Summer 2014 G&G, pp. 114–132). This emerald shows that inclusions may not always provide conclusive proof of origin, but they can still provide useful information.

Nicole Ahline is a staff gemologist at GIA in Carlsbad, California.