Hey Gothicsoul82,
Thank you so much for thinking of Dremel and we are happy you joined our community.
Finding rocks and gems along the river sounds relaxing. As for your project, we have many accessories that will work to grind or engrave on rock; however, we don't have anything that will polish rock to a high shine. Polishing stone requires progressively finer grades of sandpaper (600-1500 grit). Unfortunately, we do not have any accessories with that high grit.
Dremel accessories are used mostly for etching on rocks.
Just a fun tidbit. AJ Dremel, back in the 70’s, use to manufacture a rock tumbler to smooth out rocks. But we haven't seen one of those original tumblers in a long time.
Have a great time rock hunting!