Quartz nodule. I have many that we collected in Arizona and they look just like yours.
Most are solid but a few can have cavities inside with some minor crystallization.
Might I suggest that you have someone cut this one in half as opposed to bashing the thing with a sledge hammer. Local rockshops or rockhounds are a great place to start....If it is in fact a geode, is may well be worth a few bucks. Nix the sledge idea....No offense intended here, I just hate to see a good stone get ruined.
~Nash~
I took my rock and had it cut open. It had just two small pockets inside ; but it was a lot of fun seeing the saw in action.
I had him slice up two palm wood rocks as well.
I watch those cash and treasure shows where they have folks cut all the stones etc. I wonder if you yourself actually bought a saw and maybe have you done any cabbing?
I think I will buy some lapidary equipment in the future. I'd like to try and facet some stones.