I have one - that's been painted. Problem is one of the country's rock art academic has viewed it and think it's natural. Which is just crackers. The rock collects dust and I put it in a bath once, lukewarm water, a little detergent, and scrubbed it. Some of the paint flaked off and collected in the bath, after it was drained. I should have kept the particles - but didn't - and they had a lovely smell, like a mixture of coal dust and hemp oil. So that's what I think that paint was. The stone had been buried for many years ...