Geode

The Geode is a giant black dome shaped rock conataining a synthetic storm inside. Normally, geodes are formed by volcanic magma or balls of mud and contains crystals, but more than likely this geode was created through magic. The Geode lays at the bottom of an enormous crater which is implied by Garnet to have been created by somekind of explosion.

History
When the Geode cracked, the Crystal Gems and Steven immediately went to seal it in order to prevent the storm from escaping and it vaporizing them. When they arrived, lightning was already leaking out of the Geode and creating a thunder storm overhad. Garnet hoped Steven could seal the crack with his healing powers, but the Geode remained unchanged. Steven and the Gems then returned to the Beach House to come up with a new plan. Greg wanted to help and thereby make amends for lying to Steven and used the Warp Pad to go and fix the Geode himself. Steven went after his father and together they used a roll of duck tape from Greg's toolbox to seal the crack in the Geode.