Geode Cave
Geode Caves can be found slightly exposed to the surface, despite being hidden deep inside Nether terrain.

Features
Geode Caves are enclosed biomes of either Basalt Deltas, Crimson Forest, Warped Forest, or Soul Sand Valleys. They are surrounded by a thick shell of Blackstone, and can be found with cracks in the shell opening to the surrounding biome.
Generation
Geode Caves generate inside Nether terrain, and can be located through their exposed shell when mining.
History
This feature was added in RealismCraft 1.4: The Nether Update