The most thoroughly decorated of all the stones at Knockroe.
Image credit: ryaner
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Lovely. Makes me wonder if stones like this where ever painted in? A dash of colour would do very nice to bring out the markings more.
I think most were...and if the fixative was animal fat based then lichens could have taken hold in preference to the surrounding area, maybe even leaving traces today. Reasoning: if you want to age fresh stone the best way is to paint it with yogurt (not a lot of people know that : ) )
Paint and pigmentation sound plausible to me. There's the water technique, or lighting (see Ken Williams shots on the site page). These are quite eroded from exposure and would have been a lot sharper when carved first, and then sealed in the tomb.