Well, that's the problem. I've been playing the game in my free time over the past month, and I found five of the crystals within the first week. It's been so long that I've forgotten. I read in another post that one is found on or near a "dying drake." I think that might be the one I'm missing.
I know I've found one on an island near the cave, one in a cave on an island, one in a Slith village (I believe), and... well, those are the three I can remember. I may start a new game just to figure it out.
Thanks for the help!