I have talked about these markers before, the colours are so lush and colours just gorgeous. They are pricey BUT they last a long time and are worth it. You can use them on just about any surface. I did give these stones a coat of varnish which gives a nice sheen.
If you are looking for a fun little project for the summer with the kids, go and grab a bag of smooth rocks from Home Depot (around $12) You can use the pens right on the rocks or paint them with a solid colour acrylic and then doodle.