Have you ever heard of cave pearls? Or cave marbles?

A pearl is formed when an irritant like a grain of sand or a parasite gets into the shell of an oyster, clam or mussel. The shellfish protects itself by coating the irritant with layers of nacre, the same material that makes up the inside of the shell.

Large Cave Pearls Photo by Damien Muglia - The Cave of the Marbles - The Underground World of Caves
Large Cave Pearls
Photo by Damien Muglia

A cave pearl is formed sort of similarly. Except instead of a shellfish, you have a cave. And instead of nacre, you have calcium salts.

Cave Pearls in Carlsbad Caverns Photo by W. Tucker - The Cave of the Marbles - The Underground World of Caves
Cave Pearls in Carlsbad Caverns
Photo by W. Tucker

A cave pearl forms when a grain of sand is covered with a coating of calcite*. If it sticks to the floor, it becomes a stalagmite. But if it doesn’t form an attachment, the formation will sort of roll.  The calcite forms into a sphere** and the rolling action, over time, polishes the surface. They are usually found in a pool.

 The Cave of the Marbles

In 2005 I read an article in Awake! magazine about a cave in Mexico with some 200 million cave pearls.

“These cover the floor to a depth of up to five inches [12 cm] for approximately the next 160 feet [50 m]. No one can resist scooping up the pearls and letting the smallest ones​—as small as lentils—​slip through his fingers. The largest ones are the size of a small orange. Scraping the marbles apart to reach solid floor allows the curious to see that it is formed of evenly encrusted pearls.”

The Cave of the Marbles
Awake! February 8, 2005

The article said, “At the present, efforts to research this outstanding cave and preserve it are under way.”

So what has happened to The Cave of the Marbles since 2005? I googled. Wikipedia say; “In Tabasco, Mexico, the cave Gruta de las Canicas (Cave of the Marbles) is highly unusual in that it contains a tremendous quantity of pearls; an estimated 200 million pearls were discovered on the cave floor, in some areas to a depth of a meter or more. The mechanism for the formation of this vast quantity of pearls has not been determined.”

Geologists Study the “Marbles”

Wikipedia referenced an article from 2008. Petrology and Chemistry of Cave Pearls from Gruta De Las Canicas (Cave of the Marbles), Tabasco, Mexico by Shari Houston, Peter S. Mozley, Andrew R. Campbell and Penny Boston from Earth and Environmental Science, New Mexico Tech, Socorro, NM

They had studied the phenomena of these pearls using “standard optical petrographic and geochemical methods (electron microprobe and stable isotopes).” They surveyed the mineralogy, texture and chemistry of the Canicas pearls, but did not determine why so many pearls formed.

Where is the Cave of the Marbles?

Googling the Spanish name of the cave, Gruta de las Canicas , I found some articles in Spanish and translated them with Google. It seems that the caves are now part of Sierra State Park. It can be reached off the Teapa-Tacotalpa highway. Not for sure. Google Translation is iffy. In 2005 it was on private land.

This article, La Gruta de las Canicas al sur de Tabasco, describes visiting the cave, “As you walk through the center of the gallery, the marbles crack loudly, producing a sound similar to crushing gravel. Because of their solid constitution they do not suffer any damage.”


Amusing Planet article on The Cave of the Marbles
Amusing Planet article on The Cave of the Marbles

An article on Amusing Planet says that “The cave is located in the foothills of the Sierra de Chiapas, on the outskirts of the city of Teapa.” The article has some photos of the cave.


More about Cave Formations >

*Sometimes cave pearls may have quartz, apatite, iron, aluminum, or magnesium. But, they are nearly always calcite.

**Not all cave pearls are spheres. They can be other shapes. But, they are nearly always spheres.

The Cave of the Marbles