The mine of Naica, in the state of Chihuahua (Mexico), was opened in 1828 to mine for lead, zinc and silver ore. In 1910 a natural cave was discovered, named later Cueva de las Espadas, the Cave of Swords. The name derives from three feet large blade-like gypsum crystals (calcium-sulfate) completely covering the walls of the cave. However, what the miners discovered almost 90 years later, during the construction of a new tunnel, is even more astounding. The Cueva de los Cristales, the Cave of the Crystals, hosts the most incredible crystals ever discovered.
For crystals to grow, they need the right conditions and a lot of time. In theory there are no limits, however, perfect conditions for crystal growth are rarely met. Twenty-six million years ago an intrusion of magma pushed up the Sierra of Naica, faults formed and water started to infiltrate into the crushed rocks. Still today, 1.5 miles below the actual surface, the active magma chamber heats up the groundwater. The 131°F hot water rises up, slowly dissolving the limestone and becoming oversaturated with calcium and sulfur. As soon as the percolating water enters a natural cavity, first crystals of gypsum formed along the walls. The crystals continued to grow over millions of years, forming a forest of large blade-like crystals, crossing the entire length of the cave. The crystals can be visited and studied only in special cooling suits, as still extreme temperatures (122°F) and humidity levels (90-100%) persist inside the cave. However, recent observations suggest that the Selenite-crystals started to deteriorate since the groundwater level was lowered for the mining operations. Selenite is a transparent variety of gypsum composed of calcium-sulfate-dihydrate, meaning that its crystalline structure includes two molecules of water.