Minecraft wiki entry for Development_versions, associated text: A new feature found uncommonly underground (can rarely appear exposed to the surface close to sea level).
Contains blocks of amethyst and budding amethyst blocks, which are encased in a shell of calcite and tuff.
Also generates with a small air pocket nearby which may open into the geode.
Most commonly generates at lower levels. (y = 6 to y = 47)
Overrides standard caves and ravines, but is overridden by mineshafts.