As the summary states, amethyst geodes do not spawn in worlds created in previous versions of minecraft. In one of my old worlds I went to search for this new geode (assisted by chunkbase). (If you don't know what chunkbase is, it's a website that takes your seed and gives you a map with coordinates and a legend) I entered my seed and headed to the coordinates of a geode in a chunk that I would be generating for the first time. After clearing two chunks searching for the geode I gave up and exited the world. I then created a new world with the same seed and teleported to the coordinates of the supposed geode and it was there; viola.