Last week a friend of mine had a cyst removed from his back. It was the size of a marble, and hard to touch. Apparently when the surgeon removed it (in a five minute operation with local anaesthetic) it was a spongy little ball of muck, likely cause by an ingrown hair.
It was nothing to be concerned about, and the operation was quick and painless. He said it was DEFINITELY worth him getting it seen. It's already healing really well, with barely a mark where the small incision was made to remove it.