I have found really smooth “river rocks”, the egg shaped or oval ones like you find on river beds in campsites and middens. Many Many miles from any riverbed.
I always figured they used them for grinders or hammers.

"Allways speak the truth and you will never have to remember what you said before..." Sam Houston
Texans, "We say Grace, We Say Mam, If You Don't Like it, We Don't Give a Damn!"

~Molɔ̀ːn Labé Skýla~