Geodes are hollow rock filled with dazzling crystals. I'm at the American Museum of Natural History in New York City and behind me is an amethyst geode that was formed millions of years ago.

Unlike the giant crystal cave of Naica in Mexico, where mineralogists were able to study a still active crystal cave, the crystals in the geode of Pulpí stopped growing almost 60.000 years ago.

These human-size crystals formed in especially strange ways. New clues into ancient climate shifts reveal the possible origin story of Spain's Pulpí Geode, one of the world's largest.

The team also acknowledged that, due to their purity, the crystals comprising the geode can’t be precisely dated. Still, it is possible to indirectly deduce the Pulpí geode’s approximate age. “They ...

Amethyst is a violet variety of quartz that has been used as a gemstone for many centuries and is a key economic resource in northern Uruguay. Geodes are hollow rock formations often with quartz ...