I can't remember where I saw this idea, it is hardly novel. I collected river stones, bought test pots and used sharpie pens to write on the stones.
We also made some decoy strawberries. I read in an article somewhere that a family used decoy rock berries to misdirect birds or to hide amongst real ones to give birds a fright if they try to nibble on them. They are cute regardless and fun to make!
Again, we used a sharpie pen for detail. The stones were then sprayed with clear varnish to protect the writing. Be careful about spraying, too close and the pen may run. Spraying from 30 cm is good.