Seen these quite a lot on old ironwork underground, especially in damp conditions
My thoughts are they could be bacterial in origin; Fe-oxidising bugs colonise the artefact's rusty surface, and form a little vesicle. the bacteria make a living nibbling away at the unoxidised iron, assisted by condensing water vapour, and depositing their waste Fe3+ at the margins - which forms the hard clamshell-like nugget.
MARK