Geode

"The Geode" is a giant black dome shaped rock containing a synthetic storm inside. The Geode lays at the bottom of an enormous crater, and Garnet implies that it was created by an explosion. It first appeared in "House Guest."

History
The Geode received damage by unknown means, thus Steven and the Crystal Gems rushed to repair it and prevent the storm from escaping and vaporizing everything. When they arrived, lightning was spewing from the cracked Geode and creating a thunderstorm overhead. Garnet hoped Steven could seal the crack with his healing powers, but his attempts had no effect.

Steven and the Gems returned to the Beach House to devise a new plan.

Greg wanted to make amends for lying to Steven, and he traveled via Warp Pad to fix the Geode himself. Steven chased after his father, and together they used a roll of duct tape from Greg's toolbox to temporarily seal the Geode.

In "Warp Tour," the Geode is revisited as the Gems travel searching for the Flask Robonoids that Steven witnessed.

Trivia

 * The Geode bears an uncanny resemblance to the slow explosion in 'Akira'.
 * A geode is a round rock which contains a hollow cavity lined with crystals.