Geodes are hollow rocks that contain inward-facing crystals that are formed by numerous different processes, namely a slow flow of mineral material into pockets of air within rocks. One of the most renowned regions for amethyst mineralization and geode mining is the Artigas region of Uruguay. According to the Guinness Book of World Records, The largest amethyst geode weighs 13,000 kg (28,660 lb) and is 3 m (9 ft 10 in) long, 1.8 m (5 ft 10 in) wide and 2.2 m (7 ft 2 in) high. It is displayed in Shandong Tianyu Museum of Natural History (China) in Shandong, China.
Oh wow! I'd love to have that on display. It's magnificent. It looks like a portal to outer space, as if the rock swallowed a starry night.