I've gotten pretty good at spotting the particular kinds that interest me most. In this picture there are a couple (click any of these photos to see them larger):
There's a nice little piece of rosy quartz partly hidden in the upper center, a little to the left, and a piece of milky quartz in the lower center.
Polishing requires patience, though. It takes about three weeks to polish a batch of Lake Michigan stones, and that's speedy. Most of the rough tumbling has already been done by the lake itself.
A big bowl of polished rocks is fun to run your hands through.
But many rocks can be candidates for my tumbler, not just quartz. I just have to see them when they're wet to know what they will look like with a permanent sheen. I like rocks that are plain or stripey, perfectly smooth and round or oddly shaped.
I keep an eye out for fossils, too. Here are a couple more pieces of honeycomb coral. I didn't keep them, though.
Can you guess which one it is?