Amethyst geode

An amethyst geode is a structure-like terrain feature found in the underground of the Overworld. Amethyst geodes contain smooth basalt, calcite, and are the only source of amethyst items and blocks.

Generation
Amethyst geodes generate between Y=-64 and Y=30 regardless of whether "generate structures" is toggled on or off. Each chunk has a $1/24$ chance to generate a geode. They override only caverns and mineshafts. Sometimes they are exposed on the surface, such as on beaches and underwater in oceans.

Structure
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, and as such are often obstructed by them, except in cases where the crack intersects a cavern.