Lab Notes Gems & Gemology, Fall 2021, Vol. 57, No. 3

Fossilized Shell Consisting of Emerald

Shell outlines with green emerald grains are clearly seen in these fossilized shells.
Figure 1. Eleven semitranslucent to opaque green and brown fossilized shells. Most of these specimens show clear shell outlines with green emerald grains. Photo by Johnny Leung.

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.

Pyrite is commonly associated with Colombian emerald.
Figure 2. Well-formed pyrite is a common accessory mineral associated with Colombian emerald. Photomicrograph by Johnny Leung; field of view 14 mm.

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).

Two fossilized shells show a clear shell structure and scattered pyrite crystals, respectively.
Figure 3. Clear shell structure and internal banding are shown in the X-ray radiograph (top left) of a fossilized shell measuring 22.06 × 14.85 × 10.32 mm and weighing 14.89 ct (top right). Scattered pyrite crystals (yellow in the colorized image on the bottom left) are deposited in a fossilized shell measuring 13.91 × 11.89 × 9.32 mm and weighing 5.07 ct (bottom right). Top images by Johnny Leung and Cheryl Wing Wai Au; bottom images by Ching Yin Sin and Emiko Yazawa.

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.

Ching Yin Sin is a staff gemologist, and Xiaodan Jia is supervisor of colored stone identification, at GIA in Hong Kong.