As far as I am aware, the crystals are simply used for decoration. They have no specific purpose nor can the players interact with 'em aside from usin' their own imagination.
Of course, one can imagine 'em to be of importance but otherwise, they're simply there to give the public maps a bit of a whimsical streak.
Also, methinks this thread belongs in the Game Help section since it isn't much of a discussion. Movin' it there.