Small, hard bumps on my scalp??

This is something I've wondered about for years. I have several small (pebble sized), very hard bumps on my scalp. I first noticed one when I was a teenager. I'm now 30. They don't hurt, they don't itch, they don't bother me. But I have no clue WHAT they are. Moles? Warts? Anyone ever had something similar?