Is a Coticule base slate or belgin blue water stone

[niftyshaving]

Is a coticules base always slate or belgian blue coticule
and more importantly how does someone tell the difference?
Does it matter?

I ask because on another thread there was a wonderful
set of pix of a stone and the dark side looked a lot like slate
to me but I only own a stone with a slate base as
a reference and pictures as good as these were do not tell
the full story.