Here is the stone Rupert is talking about...
http://www.themodernantiquarian.com/post/65688/bryn_celli_ddu.html

but there seems to be no more info about what it is. Surely you would have to take some stone off for investigation to prove one way or the other, which of course means defacing it.

Coflein gives it as 'tomb guardian' phallic symbol stone, not sure it is the same one but fascinating none the less......

http://www.coflein.gov.uk/en/site/93827/details/BRYN+CELLI+DDU+CHAMBERED+TOMB/

The video bit is here and well worth watching (again).

Rupert makes a couple of points that are interesting: 1 that it’s not structural as it doesn’t touch the ceiling [although it may have moved or been moved]. 2 that wood can fossilize (in the right conditions) in under a century. It could therefore have been used in its wooden state [in a building or a bridge for example] then fallen into a bog with a high acid/silica content and been fossilised. ‘When it was rediscovered it would have seemed like a thing of absolute magic. To Neolithic man wood symbolised life and stone death.’

Michael Bott posted the find in a thread on TMA back in 2009 (where they the first to suggest the wood/stone-life/death theory and also that the stone might be fossilised wood – wow!).

Looking forward to any info you can bring back with you CARL.

if you look around inside the mound there are other stones that look exactly the same as the stone we are talking about, also the back and top of the stone look nothing like wood.

i do agree that the front of the stone looks like wood, but the rest of it doesnt.

id love to be proved wrong though