I've heard from the ragged rascals, I hope they don't mine me posting these snippets
PeteG

"...it looked like igneous rock.

We have pics of one of the smaller stones in the passage (the
"betyles") when it was displaced:-
http://www.megalithics.com/wales/brynceli/celnicl.htm

We noticed the striated "bark-like" surface of this smaller stone had
similarities to the larger chamber stone. Given that these stones
were set as focal points, it is likely that were specially selected
for this appearance.

We also noticed that one of the stones in the wall of the passage
grave La Hougue Bie had a strange "organic" tree-like appearance, and
this stone was concealed in a strange unexplained three stone
"baffle" niche constucted in chamber. We have a pic of this stone on
our La Hougue Bie page:-

http://www.megalithics.com/europe/jersey/bie/biemain.htm

It's about halfway down, above the cup and ring diagram, obviously
not a fossil, but unusually shaped and specially placed with
restricted visibilty. Interestingly, La Hougue Bie also contains
betyle stones."