What a simple and fun project! These trivets are a natural heat insulator and will protect your counter tops. Cut a piece of felt purchased from a hobby store into the shape you desire. Hot glue stones onto felt (also can be purchased from a hobby store) and your done! You can also cover a placemat or table runner with stones for an organic look.
Source: http://www.vivaterra.com/river-stone-trivets.html, http://www.vivaterra.com/multi-color-river-stone-placemats.html, http://www.vivaterra.com/white-contour-dinnerware.html