I did some fun experiments last night on the rocks. Instead of drawing on them, I tried engraving two rocks with my rotary tool and an engraving bit. The lines have a different quality; they are rougher and muted, a more primitive look which I like. I colored one with color stamp pads and left the other one untouched. Over all, it's not technically difficult to engrave on rocks (though obviously I need more practice); the key is to hold the power tool firmly, but at the same time be light on your hand and don't press into the rock and let the bit do all the hard works.
Below are a few more doodled rocks and two paper mache boxes I did over the past week. Just thought I'd like to share. : )