BosnMate , I've really have been enjoying your post here for the past week or two . Like you and your family , I collected here in So. Cali , and it's not often were treated to seeing artifact's from around here .
The Native Americans along the Cali cost have left a few odd artifacts behind,.. not the"odd few" ,.just thing's a bit unique to there culture .
It's probably not a bad call ,.labeling this artifact a Nutting-stone ,.if so, think of it as a little anvil to hold shell fish , nut's and the like for a crack'n ,..and for other task that that fingers need not to be in the way of ?
But , there are a few other Cali "oddity's" , that it could be, the size and symmetry lends it's self toward a Donut- Stone , which is a type of Cali discoidal , and there is the cogged-stone , both of which the use is not understood,.. so ,labeled "Ceremonial".I'm not saying this is a Cogged-stone , and the typical Donut -stone does have a hole ,..but

cap-stone,maybe??,..nice little artifact for sure !!
Oh,...BosnMate , have a look on the sides of your metate's ,you might find a bit of a pit polished into it ,..from shelling those acorns ,...one at a time ...
thanks for the look ,.Blindpig