Quote Originally Posted by AlanII View Post
Could well be. I have stones where the BBW seems to have leached into the cuticle though. Is BBW manganese? Surely not. Genuine question
No the BBW is just a sediment layer naturally turned into
slate by heat and pressure. It does have a different composition
than the sediment that becomes garnet rich coticle. I
think of the site as a deposit of metamorphosized layers of mud stone
and silt stone.

You might find a more precise answer at Barts site.
http://www.coticule.be/

Perhaps:
http://www.coticule.be/the-cafeteria/topic/13.html
I do see a new link to
ArtisanShaving.org • Index page that may find a person
willing to answer a question as coticule.be seems to
be closed