“C” is for Cookie and “G” is for Geode

“C” is for cookie and “G” is for geode. Gemstones are well known for their ability to mimic familiar themes, but the rock that recently launched a thousand online posts resembled one of the most recognizable children’s characters, the Muppet known as the Cookie Monster (figure 1). The uncanny likeness set the internet ablaze with reports from news sources around the globe. A video of the geode featuring Cookie Monster’s signature tune, “C is for Cookie,” was uploaded to Facebook by its owner, gem collector Mike Bowers, on January 16, 2021. The viral clip reached more than one million views just a week later. All the attention left many wondering, “How could a rock like this form?”
The discovery of the Cookie Monster geode, reportedly from Brazil, is a combination of geologic design and pure luck. It is composed of banded layers of microcrystalline quartz, commonly known as agate. The scientific processes required are complex but can be simplified into some general steps (J.D. Winter, Principles of Igneous and Metamorphic Petrology, Pearson Prentice Hall, Upper Saddle River, New Jersey, 2013). This type of geode occurs as a secondary formation when gas bubbles trapped in volcanic rocks, known as vesicles, are filled with silica-rich fluids. Over time, the fluids deposit concentric layers of microcrystalline quartz along the interior of the vesicle. The accumulation of these layers reflects the variation in chemistry and changing conditions of the silica-rich fluid. These fluctuating conditions can result in a banded formation of the microcrystalline quartz, thus giving rise to an agate. The final siliceous fluid in the last stage of formation allowed for a large central druzy quartz–lined cavity that represents the “mouth” of the Cookie Monster (figure 2, left).

Cutting the rock at the perfect angle to reveal such a startling semblance was a matter of sheer chance, for the internal structure of a geode is not known until it is split open. Any slight change in cutting direction might have resulted in viewers not experiencing the phenomenon of pareidolia, the tendency to assign familiar shapes or faces to abstract forms (Winter 2007 Lab Notes, pp. 363–364).
The color in blue agate is created by scattering of light; when the particles of silica are smaller than 400 nm, blue light is reflected back (G. Rossman, “Cause of color in agate and chalcedony,” Seventeenth Annual Sinkankas Symposium, April 24, 2021). When the rock was exposed to long-wave ultraviolet light, it revealed strong blue fluorescence (figure 2, right) consistent with the fur color of the famous Muppet. Also of gemological interest was a microscopic feature: Below the quartz-lined “mouth” was a small inclusion of yellowish limonite, a hydrous iron oxide, resembling a cookie crumb (figure 3).

While geodes can be millions of years old, the Cookie Monster character only emerged within the last century. The Cookie Monster, originally created as a prototype in 1966 by Jim Henson, debuted on the American educational children’s television series Sesame Street in 1969. The name was taken from his incessant obsession with the baked goods. With Sesame Street still on the air today, after 51 seasons, Cookie Monster’s fame is sealed in pop culture.
It was a fortuitous circumstance that the formation of the geode resulted in such a remarkable specimen. As GIA analytical microscopist John Koivula likes to say, “Mother Nature cooks in a dirty kitchen.” Who knows what she will cook up next, but one can hope it has us all cracking open a smile once more.