These heavy metallic blobs keep turning up in my yard as heavy rains wash away exposed dirt.
Googling has turned up hematite concretions as the most likely ID, after a wild-goose-chase with manganese nodules.
They are non-magnetic.
First chance I get I'll have one band-sawed in half to see the inside detail, but I doubt I'll find anything different from the fragment of a larger one on the right--although it suggests that they might be hollow. I doubt it, though.
I like using words like concretion and nodule. Makes me sound smart.