My thought is a hickory nut too.

They have a thick outer covering to get to the shell:
http://blog.growingwithscience.com/2013/01/seed-of-the-week-shagbark-hickory/

We don't have pecans in this area, but I am under the impression that their shell is very fragil, paperlike:
http://www.google.com/search?site=imghp&...320.SiQxc9lok7M