I think Chris is on the money here. I have a bunch of old natural and glued Coticule combos, and many of the old ones get a brownish tinge from age to the BBW side, and even the Coticule side. I don't know how or why. I lap the stones to make them usable rather than for cosmetics, but lapping definitely makes a cosmetic difference. All of my lapped BBWs vary from deep purple to deep blue, like the newly mined ones.
My best Coticules actually tend to be the newly mined ones, most of my vintage Coticules are fast cutters but not-so-great polishers. But I just love the unusual sizes and fat thickness of the old ones. So, I try to find rough old ones for cheap, and rescue/restore them to their rightful place of razor honing glory. Vintage purists, please don't look at the photos below, it'll just break your heart.... :rolleyes:
Here's an old 2.5 X 5 X 1" "Barber Coticule" before and after (intense) restoration lapping. You can see the brownish tinge before lapping, and the plucked-fresh-from-the-mine purple appearance after lapping. I love this stone, it's actually a natural combo with a long-ago-repaired gap, but it looks like a glued one at this angle:
http://i76.photobucket.com/albums/j2...tiUnlapped.jpg
http://i76.photobucket.com/albums/j2...CotiLapped.jpg