Any amateur geologists on the forum?

[user 423]

My friend often walks his 5 acre lot near Sierra Vista, AZ and has found about 20 of these small,round stones.

They weren't found in any one location but were scattered around most of the property.

They're not old marbles since they aren't perfectly round and they vary in size by 10-20%.

Any guesses on what they might be? Or have any of you rock hounds seen similar?

 

[Frood]

I would guess some kind of concretion... but take to a rock shop for an opinion and maybe get one cut in half with a lapidary saw to see if it has a core/seed in the middle. Concretions can be valuable too depending on rarity, beauty, etc... Maybe take it to a local university geology dept...