A volcanic bomb, picked from the slopes of Mount Etna. (Good job they didn't ask me what it was at airport security)
A tooth from a buffalo. Picked from its jaw, and with remnants of its last meal in the ridges.
A roller bearing that is square - pulled from the sump of a failed gearbox. And by square, I mean square, as in 4 equal sides with 90° angles between adjacent ones.