We,ve absolutely no doubt that the stone was chosen for its appearance - as Rupert says in the film, it would have appeared as magical to whoever placed it there. Interestingly, some of the stones used to build the monument itself are 'wood-like' in appearance, most notably the lintel and stones around the entrance. However, those stones are definitely stone whose geology lends the woody surface texture. Also those stones are slabs - that is - the stone has naturally fissured to reveal the texture.

The pillar, on the other hand, is cylindrical. And the texture seems to be a surface texture - not an outward expression of the matrix of the stone. It's one of those things that is hard to discuss without us all being in the chamber together and thrashing it out! Photos or film just does not seem to convey the sheer 'woody' presence of the thing.