Fossilized Shell Consisting of Emerald
Fossilized shells can be replaced by various types of gemstones, such as quartz and chalcedony (Spring 2014 Gem News International, p. 77), opal (A. Cody and D. Cody, The Opal Story: A Guidebook, Melbourne, 2008), and demantoid garnet (Winter 2013 Gem News International, pp. 257–258). In rare cases, emerald may also participate in the petrification of the shell and form pseudomorphs.
Recently, the Hong Kong laboratory received 11 fossilized shells composed primarily of emerald, measuring 13.00 × 8.20 × 6.16 mm to 24.54 × 16.72 × 12.57 mm and weighing 3.22 to 20.63 ct (figure 1). Most of them preserved the distinctive gastropod shell outlines, with different degrees of weathering.
Under magnification, numerous small light green to green anhedral emerald crystals contained very fine fluid inclusions associated with well-formed brassy pyrite grains (figure 2), which is one of the most common mineral inclusions in Colombian emeralds (S. Saeseaw et al., “Geographic origin determination of emerald,” Winter 2019 G&G, pp. 614–646).
An X-ray radiograph further revealed the spiral skeleton of the shell and scattered pyrite crystals (figure 3). The polycrystalline emerald was deposited evenly throughout the specimens, indicating complete replacement.
Fossilized gastropods were reported from the Matecaiia tunnel of the Gachala emerald mine in Colombia (P. Vuillet et al., “Les émeraudes de Gachalá, Colombie,” Le Regne Mineral, No. 46, July/August 2002, pp. 5–18). Gachala is not a principal emerald mining district but can produce high-quality material (D. Fortaleche et al., “The Colombian emerald industry: Winds of change,” Fall 2017 G&G, pp. 332–358). It is located on the Lower Cretaceous fossiliferous sedimentary rocks of the Eastern Cordillera Basin (B. Horton et al., “Construction of the Eastern Cordillera of Colombia: Insights from the sedimentary record,” in J. Gómez and D. Mateus-Zabala, Eds., The Geology of Colombia, Chapter 3, Vol. 3, 2020, Servicio Geológico Colombiano, Publicaciones Geológicas Especiales 37, pp. 67–88), where pyrite and emerald crystallized during the circulation of hydrothermal mineralizing fluids in black shales (G. Giuliani and L. Groat, “Geology of corundum and emerald gem deposits,” Winter 2019 G&G, pp. 464–489) and subsequently precipitated to form the fossilized shells.