Geode

From the press release:
Heads Up, Rock Hounds: Palace Gem & Mineral Show Returns

New Mexico History Museum

Exhibitors at the Palace Gem & Mineral Show can tell you what kind of rocks to look for and how to carefully open them to reveal geodes like this one. Photo by Natalie Baca, New Mexico History  Museum.

Note: Representative image at left is often cropped for display purposes. Downloaded high-resolution images are not cropped.