Nice. A couple of years ago my sister got me some garden markers that are wide white plastic (about 2" wide) that came with a grease pen. Write the plant on the marker and at the end of the season a little rubbing alcohol and it comes right off and ready for next year. I like the rocks for items you plant year after year.
@chelloveck what kind of paint are you using? I've used nail polish and oil based paints and they rub off after a few weeks. I'm always looking for a better solution
Just taking a guess here as I don't do this, yet, but you might need to put some sort of shellac or varnish on the rock first, or paint the rocks with a primer. That would give the paint something to stick to. Also, make sure the rock is cleaned really well. Here's something I found online:
thank you @oldawg a brilliant simple painting solution for this BRITE-MARK Medium Tip Paint Marker, White: Construction Marking Tools: Amazon.com: Industrial & Scientific
@Ganado Not my pic, though my father used them in lieu of how to vote cards when he ran for political office (unsuccessfully). It became something of a family standing joke, saying that of all the candidates, he was the only one with the courage of his convictions. If he did not live up to his election promises his electors had the option of stoning him with his own how to vote stones. Further to your question, you may be able to extend the life outdoors by giving the surface of the stone a light dusting of flowerpot sealant, The kind used to seal unglazed flower pots.
No matter how you paint them, add a clear coat of polyurethane, and it will stay in place, and clean up with water. I used to plant things designed to improve memory, but I could never remember what they were, or where I planted them....