Micro-World Gems & Gemology, Summer 2019, Vol. 55, No. 2

Iridescent Tabasco Geode

Tabasco geode featuring greenish blue botryoidal chalcedony.
Figure 1. This unique Tabasco geode from Mexico’s Zacatecas State, containing greenish blue botryoidal chalcedony, measures 24 mm in the longest dimension. Photo by Diego Sanchez; courtesy of Marquez Mining.

Tabasco geodes are a small geode variety named for the area where they are mined in the Mexican state of Zacatecas, near the city of Tabasco. Typically, these geodes are lined with water-clear drusy quartz, but the author recently examined one that was filled with greenish blue botryoidal chalcedony that also showed iridescent colors (figure 1). The cause of the colors was not immediately apparent, but microscopic examination revealed clues. The outermost layer of chalcedony had not firmly adhered to the underlying botryoidal mass. This delamination allowed an air gap along the interface of the thin outermost layer of chalcedony and the botryoidal mass of greenish blue chalcedony underneath. This air gap, in addition to the transparent nature of the very thin chalcedony shell, created thin-film interference colors along the interface of the chalcedony shell and substrate (figure 2). This explanation was supported by the observation that where areas of the delicate shell were damaged, the iridescence was not present. This Tabasco geode is one of the most visually interesting geodes examined by the author.

Iridescent thin-film interference colors seen on a thin layer of chalcedony.
Figure 2. The Tabasco geode hosts greenish blue chalcedony, with a thin chalcedony shell that was responsible for the iridescent thin-film interference colors. Photomicrograph by Nathan Renfro; field of view 1.83 mm.

Nathan Renfro is manager of identification at GIA in Carlsbad, California.