Another thing you could try is making a rubbing from it (like a brass rubbing) or using water-based ink/paint to make a print directly of the stone. I'm not sure that either will help with the lichen, but it won't cost much to try!
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Mike
Pentax K5ii & Panasonic G5 user (with far too many bits to list)
Member of North Essex Photographic Workshop
Also online with PentaxUser.co.uk, Flickr, MU-43, MFLenses...
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