Minecraft wiki entry for Development_versions, associated text: Grows from budding amethyst, which is found in amethyst geodes.
Starts out tiny, but grows into medium after a while, large after even more time, and eventually becomes an amethyst cluster.
Unlike crops, each growth stage is a separate block instead of a block state.
Drops only when mined with a Silk Touch pickaxe.
There is a total of 3 variants (stages).