Researchers from the University of Göttingen in front of a giant amethyst geode in Uruguay. Geodes like this were discovered in cooled lava flows dating back to the original breakup of the Gondwana ...

Amethyst geodes—some stretching 5 m long—dot the ancient lava flows of Uruguay’s Los Catalanes mining area. The geodes contain calcite, colorless quartz, and agate, another quartz mineral.

Amethyst geodes (some over 16 feet high!) are located in cooled lava flows. These ancient lava flows can be traced to the breakup of the supercontinent Gondwana some 134 million years ago.

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.