Why would you need a tutorial specifically for a ball pan? The geode would be made the same as on a round, square or stacked cake. Any tutorial should work for you.
Since geode's are not completely round, I would stack a half sphere, the carve divots here and there to make it look more organic. Then carve out the space for the crystals on the top. There is sometimes a border around the crystals, so you could mix up a few strips of fondant in various colors (check a photo for reference) and press it around the edge.
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