Not sure?

[Guest]

Piece found Feb-2013 in Bexar Texas

[Hoss]

OK David as a convenience to you and other here I have moved yet one more post for you. I am not trying to scare you off but feel it is important to give you some guidance. Give your posts some time to show up before posting an identical post. Make a post and refresh your browser. look at your post. Forget too write something? Click the edit key and post what you forgot! 
This could be a concretion which had a void within filled with limonite? I could be wrong. Native Americans would seek these out and use that material inside as a pigment

[rmartin]

I agree with Hoss, this looks entirely natural to me. It could have been used as found but I see no evidence of that.