Minecraft wiki entry for Amethyst_Geode, associated text: Amethyst geodes consist of three layers: an outer layer of smooth basalt, a middle layer of calcite, and a hollow layer of primarily amethyst blocks, with 8.3% replaced by budding amethyst blocks instead. Amethyst crystals generate within the structure on budding amethyst. Geodes have a 95% chance of generating with a crack, exposing its inside. Caves, aquifers, and canyons cannot cut into geodes,[1] and as such are often obstructed by them, except in cases where the crack intersects a cavern.