My husband has the same thing, in the same spot reoccurring on his arm. It sort of heals for a bit, then scabs up in a way and protrudes like a small callus. We've examined the little "stones" he pulls out of it, which seem to be little hardened crystals. Sometimes 2 or 3 will be in there after it's left alone a while. He was told it they were cholesterol deposits that grow in sort of a viral wart area. He's had other types of warts on his hands and feet, supposedly related to the same virus that most of us carry. Hope that helps.