very unusual coticule

Has anyone ever seen a vintage coticule with a natural – i.e. wavy – seam, but where the other side isn't a Belgian blue but is rather some sort of natural black/charcoal gray slate-like rock?

This coticule is really odd. Very creamy appearance on the top, then from the side the yellow is in wavy strata of different shades, then a sharply defined and very wavy line between that and what I swear isn't Belgian blue but some kind of natural slate.

Is the coticule always found adjacent to the blue, or are there other geological formations in the coticule deposit?

I have seen such photos, Curtis

Btw, I think that the Blue was a relatively *recent* discovery... Especially the fact that it can be used for honing. I am sure there are also other geo formations adjacent to coticule veins

Cheers
Ivo