It's called the Wood Stone:
http://www.themodernantiquarian.com/site/6999/tooting_bec_common_stone.html
I wonder if this adds another aspect to the popularity of jet objects from the bronze age. Before you work it you can often quite clearly se the woodgrain.
Unless otherwise stated, the copyright in each work is the property of the respective contributor. For usage or questions, contact [email protected].