I think it's radioactivity. From what I learned in 5th grade, the hardness of a mineral can be a major indicator, as a diamond is the strongest mineral, and talc is extremely easy to break down (I think it's Talc at least). A quick google search showed no mention of radioactivity. Also radioactivity just doesn't sound right compared to the other answers.
It's been a while since I've gone over it so I could be wrong.
