This is impossible to answer, however generally a geode can be just a concretions of sand and a few other things. The crystals inside could be quartz or amethyst, so you start to see the problems. Let's continue, quartz has many qualities, one of which is color and purple quartz is amethyst. Other qualities are purity, lack of inclusions and imperfections, color. So you could have a geode with amethyst inside of a rare hue. Generally though Amethyst and garnet are considered gems, quartz and geodes are more often considered rocks.