It sounds like a dark tourmaline that will most likely stay dark no
matter what you do. None the less, you might be able to lighten it
abit. I would say one thing might be to cut the side pav facets to
the proper angles first and see what that brings. Then bring them
in a bit at a time to see if you can improve color. Remember, you
are not cutting this stone for brilliance or scintilation...it is
long past that.
I would not bring the long end facets in. Tourmaline tends to just
blacken out if you do that. Leave the ends fairly open. After
these efforts, if the stone has not improved, you will just have to
accept it for what it is...a large dark stone. I have a
beautifully clear but dark forest green stone that no amount of work
would improve. I just keep it as a reminder that there are limits
to what a cutter can do!
Cheers from Don at The Charles Belle Studio in SOFL where simple
elegance IS fine jewelry! @coralnut2