Crystal Cave is a location that appears in the Sonic the Hedgehog comic series and its spin-offs published by Archie Comics. It is a massive geode located deep within a mountain which was originally a victim of being excavated, solely to gather the purple Chaos Emerald hidden inside.
Essentially an underground geode, the Crystal Cave is a colossal and deep cave with nearly every surface covered in unusual large quantities of purple gypsum selenite crystals which are large enough for a person to stand on. As such, there are no safe surface to walk on inside it. Because of its formations, the Crystal Cave is very beautiful and considered a natural wonder.
Sometime before Shattered World Crisis, Clove the Pronghorn and Cassia the Pronghorn let an operation inside the Crystal Cave where they had Badniks excavate the cave to search for the purple Chaos Emerald hidden deep inside, but the operation was put on hold due to the cave's terrain. Later, after the planet broke apart, Miles "Tails" Prower, Sally Acorn, Antoine D'Coolette, and Nicole the Holo-Lynx sneaked inside into the cave at the dead-end of the tunnel excavation after temporarily deactivating all of the Badniks in the tunnels to find the Chaos Emerald. There, they were awestruck by the sole sight of the Crystal Cave. While Tails and Nicole found the Chaos Emerald, Sally and Antoine were amazed by the beauty of the cave. Realizing that Eggman would tear down the cave in search for the Emerald in vain, Sally decides to let Eggman know they were at the cave to preserve the geode, though the group had to convince the logical Nicole it was the right thing. As such, the three Freedom Fighters fought against all of the Badniks and openly showed themselves on Eggman's cameras in the cave to show they had the Emerald. Despite an E-1000 managing to delay their mission, the team were able to destroy the machine and could leave with all of the Badniks destroyed and the Crystal Cave protected from any harm.