This rock painting was done by my daughter. She used the paint brush to get these effects, it is probably the way she handled the brush to paint it :)
acrylic paint or fabric paint
These rocks can be found by a river or beach, it is also sold in craft shops. For these stones we used acrylic paint. I didn't dilute the paint much, but if needed use water sparingly.
Paint the first coat and then decorate only when the first coat is dry. Children can take a break between painting until the first layer dries then they can put designs.
I put fabric paint on the list because it can be used instead of acrylic paint. I have painted rocks using fabric paint which also last long on the rocks.