Although this may seem like a very trivial question, I still do not know how to interpret this. Several (sonic) spells and effects, like shatter or shout mention different effects on crystalline creatures.
And yet, there is solely one creature in the MM, the salt mephit (plus possibly animated glass objects), which mentions having such trait, and even then, it is just in the fluff text.
Now I am having trouble believing that this text about crystalline creatures has been added to sonic attacks just for the sake of the salt mephit, which an average group of adventurers might encounter once per career. I also feel that sonic attacks having extra effect on transparent (the other meaning of crystalline, according to ldoce) or invisible creatures, regardless of composition, doesn't make sense.
So how are crystalline creatures defined? Is there more of them in the core MM? Is there more of them elsewhere? When should the players be able to use it?