You can see a picture of the Porosphaera beads (mentioned above by Gladman) in this article by Christopher Duffin: ‘Herbert Toms (1874–1940), Witch Stones, and Porosphaera Beads’. Herbert Toms collected lots of folklore about naturally perforated stones. It’s in Folklore v.122, April 2011.
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That is so cool... proof that everything that glitters - at least metaphorically speaking - need not be gold. Far from it.