Minecraft wiki entry for Budding_Amethyst, associated text: JAPPA has said the reasoning for the 'x' pattern on the block is because "the plane-crystals grow out of it".[1]
According to Agnes Larsson, budding amethyst cannot be obtained without commands in Survival mode because the developers wanted to encourage players to return to the same amethyst geode repeatedly, rather than mining away the budding amethyst and farming it at their home similar to the spawner.[2]
According to Cory Scheviak, budding amethysts were inspired by atium from the fantasy novel series Mistborn.[3]