How do I get my 83 year old mom to a Psychiatrist to be evaluated?

Very patient cops. They know you are aware of the problem, right? Maybe you can give them a heads up so they can send out a senior volunteer patrol or courtesy patrol for future "missing rock" calls? I know the police have "the list". Hopefully they have your contact number next to her name in case something DOES happen. Can you get her to call you first? You might have to play along by inspecting her driveway, asking her what she thinks happened, etc. You might have hit the "agree & re-direct" stage of dementia where logic doesn't work, and you have to marvel along with her than anyone stole her driveway rocks ("what is this world coming to!"), then distract her (wanna go out for ice cream/walk in park, etc).

When my dad's dementia first began manifesting itself he was very paranoid, all of these bad things were going on and he knew all about it.

I would start with your mom's primary care Dr. but don't be surprised if your mom puts on her "nice and normal" face for the Dr. The only person who really saw my dad's paranoia was me. If asked by a Dr. or other healthcare professional how he was doing he'd reply, "Oh, I'm doing fine!!!"

Her Dr. can evaluate her and maybe put her on some medication.

Is this recent behavior?