Amethyst Geode

This 12-foot-tall geode, now on temporary view in the Grand Gallery, is one of the largest in the world. Recently acquired from  Uruguay, this extraordinary specimen will be a centerpiece in the new Allison and Roberto Mignone Halls of Gems and Minerals, which are expected to open in 2019 as part of the Museum’s 150th anniversary celebration.

An ancient geological drama gave rise to the conditions for these glittering crystals to grow. Some 135 million years ago, red-hot molten magma poured out of vents in the ground, forming lake-like sheets. As one of these sheets cooled quickly, giant gas bubbles formed within the magma and were frozen into the rock as it solidified. 

This geode formed from one of those giant bubbles, called amygdules. As the hardened magma cooled, water slowly seeped into the cavity, carrying dissolved chemical components that crystallized as chalcedony and then quartz crystals on the cavity wall. Over millions of years, the quartz turned purple, becoming amethyst. The crystals built up over time, growing toward the center to create a geode.