I crocheted over some river rocks with embroidery floss. turned out a lot easier than I thought it would be. I just made a circle and then branched out with chains from there until it was big enough to cover the rock, then I decreased around the rock until I got to a stopping point or ran out of embroidery floss. if you need more details than that, here's a post that explains how to do it.
the idea was to use them as weights for napkins or paper plates outside on picnics, but I think I'll put them in a glass container until we're out and about again.
I like how the backs look, too. it's a little harder to crochet against the rock but once I realized it didn't have to be super tight I managed to make it work and stopped scrubbing up my hook.