rocks rocks rocks

OK, so, my kid loves rocks. A lot. She has always been that kid who loves collecting different rocks and hiding them in her pockets and lunchboxes and in her locker at school until it becomes a problem, all the special, special rocks. Recently, when I said "I need a few bagel-sized rocks for this project I'm doing", she JUMPED UP from the milkshake she was drinking to throw on her shoes and excitedly run outside to gather rocks.

This means: rocks are better than ice cream. I'm just saying.

Obviously, the next step is to get a rock tumbler for her birthday. However, I know nothing about them, and there is a dizzying array of rock-tumbling products on the market in a variety of apparent quality levels. There are plenty of articles out there on the subject, but since I've never owned or used one of these devices, I thought I'd ask you! Have you ever polished a rock? Got any advice for a parent trying to please a tiny rock nerd?