I like the sound of the 'ragged rascals' and always warm towards people who qualify their knowledge and enthusiasm with the words 'we're no experts'.

Anyway, this got me on the trail of igneous rock and I discovered that obsidian, which was highly sought after and widely traded for axe heads, belongs to this group.

Lots about rocks here:

http://geology.com/rocks/metamorphic-rocks.shtml

Rupert asked yesterday if I had found or bought my piece of fossilized wood; I bought it in a crystal shop, it is very beautiful and I (carefully) handle it often .....

Sorry, I have digressed slightly, fascinating thread, of great interest.

I'm not sure exactly which stone they are referring to, Mike and I noticed quite a few woody looking pieces but they were definitely all rock. So maybe I missed it...dunno:-)