I was thinking how cool it would be to have different types of ore spawns for the same mineral type.
What I mean is that you would find pockets of iron(as an example) in stone, but also be able to find crystal forms of iron growing in caves.
This could happen if an ore spawn generated partially in a cave.
I know crystals have already been suggested, I just thought I'd add an idea for some crystals other than the usual gems. (Though those would be awesome too.)