Since the crust is on the outside of a crystal, the crystal below must be even older, García-Ruiz explained. The crystals must be at least 60,000 years old, the team found, because that was the youngest age of a bit of carbonate crust growing on one of the largest crystals in the geode. Ultimately, the researchers narrowed down the geode's formation to a window of about 2 million years (not bad for the 4.5-billion-year-old calendar of geologic time). The team's driving question: Where did the calcium sulfate in the Pulpí Geode come from? The researchers analyzed and radiometrically dated rock samples around the mine for seven years to figure out how the area had changed since its formation hundreds of millions of years ago. 'So, we were required to make a cartography of the entire mine to understand its very complicated geology.' 'We had no idea what happened,' García-Ruiz said. (Image credit: Alexander Van Driessche/ Creative Commons 3.0) This means they do not contain enough uranium isotopes to perform radiometric dating, a standard method of analyzing how different versions of elements radioactively decay to date very old rocks.Ī researcher is dwarfed by the telephone-pole-sized gypsum pillars in Mexico's Cavern of Crystals. On top of that, the geode's gypsum spikes are incredibly pure - so translucent that 'you can see your hand through them,' García-Ruiz said. Uncovering the history of that 'Sistine Chapel of crystals,' as García-Ruiz called it, was made easier by the fact that the crystals were still growing in the mine's humid bowels.Īt Pulpí, however, the mine was completely dry, and the geode's crystals had not grown in tens of thousands of years.

In 2007, he published a study on Mexico's fantastical Cave of Crystals, a basketball-court-size cavern of gypsum beams as big as telephone poles buried 1,000 feet (300 m) below the town of Naica. García-Ruiz is no stranger to giant crystals. 15 in the journal Geology, García-Ruiz and his colleagues attempted to shed some new light on the mysterious cave by narrowing down how and when the geode formed. The crystal columns at Pulpí are made of gypsum - the product of water, calcium sulfate, and lots and lots of time - but not much else has been revealed about them since the geode's unexpected discovery in 2000.

Natural amethyst geode