Geode

The "Geode" is a giant, black, dome-shaped rock containing a synthetic storm inside. The Geode lies at the bottom of an enormous crater held together by a piece of duct tape. Garnet implies that there was almost an explosion at the location. It first appeared in "House Guest".

"House Guest"
The Geode was cracked by unknown means, thus Steven and the Crystal Gems rushed to repair it to prevent the storm from escaping, and in turn, 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 of saliva, but his attempts had no effect.

Steven and the Gems returned to the Beach House to devise a new plan, meanwhile, Greg wanted to make amends for lying to Steven, so 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.

"Warp Tour"
The Geode is revisited as the Gems travel searching for the Flask Robonoids that Steven witnessed. It is revealed that the duct tape from "House Guest" is still holding in the storm.

Trivia

 * The Geode bears an uncanny resemblance to the slow explosion in Akira.
 * It also bears a resemblance to the Ukrainian town destroyed by Crona from Soul Eater.
 * The Geode is the first object which Steven failed to heal.