Amethyst Geodes at the American Museum of Natural History

Millions of years ago, in present-day Uruguay, gas bubbles became trapped in magma as it hardened to rock; over time, these cavities morphed into amethyst-quartz-lined geodes. A dazzling twelve-foot-tall example (pictured, in detail, above) greets visitors to the American Museum of Natural History’s newly redesigned Mignone Halls of Gems and Minerals, a showcase of some five thousand specimens from ninety-five countries, which opens on June 12. Advance tickets to the museum, via amnh.org, are required.