Pitted or porous surfaces may be hard to identify ... but their reverse side may provide the needed clue

[NFLfinder]

are there clues here ... that allow you to make an identification

[Rockwood]

This may sound odd, but what color are these ?

[NFLfinder]

all black

[Rockwood]

They don't look like fossils to me. 

But then again, they don't look black to me either.  

[Plantguy]

Last photo resembles a beat up glyptodont frag.

Regards, Chris