I have had this happen since I was about 17 or so. The only other person I know that has had this happen was my grandfather. I was telling him about it and he said he thought is was small pieces of plaque. I didn't realize at the time, but he told me they had a very foul odor to them if you smashed them. Sure enough, it is the worst smell I have ever smelt. They obviously aren't as hard as a rock, but they are hard. They have a yellow tint to them. I just wish I could figure out what is wrong and what it is. Have you had this happen to you or do you know someone who has?