Okay leaving the practical aside for one moment and exploring another avenue... if this is a fossilised tree/stone? Why is it here within a tomb... first of all is'nt it near the route of the Irish into Wales, colonists as well, and of course the 'sacred' tree that sits at the heart of the celtic settlements. There's a big time gap, but mythology floats down through the ages and if other tombs were decorated in particular fashions you may have the People of the Tree, like you have the People of the Sun/ammonite at Stoney Littleton.. there is another named stone at Lough Gur, which had at one time resided at the bottom of the lake there..Michael Dames - Sacred Ireland has the story...

http://www.themodernantiquarian.com/site/2259/stone_of_the_tree.html