All very interesting to hear and see people's experiences!
I'm going to offer a slightly different take though. I've only had 8 or 10 CotixBBW stones, so I'm sure others have more experience, but here are my general impressions (perhaps 'generalisations'). FWIW - most of the stones I've had have been natural combis, maybe other BBW can differ (?)
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Yellow coticule has more variation than BBW. I have a yellow coti that is notably coarser than any BBW I've had, and another that's finer than any of them. But as an average I think they're actually quite similar. BBW will finish a razor pretty happily.
Yellow coticule is generally quicker than BBW. You do get some very slow yellow stones, but more often than not BBW is slower for sharpening. Both stones I'd consider quite quick in comparison to most jnats I've tried at a similar level.
While I'm sure that the speed is in part to do with the amount of abrasive garnet in the stone, I'm also certain it's to do with the shape. If you look at how aggressive and deep a jigane scrath pattern off yellow coticule is compared to BBW you can see this quite clearly. As a substance BBW is more argillite or slate-like; the particles in it are more rounded I think, and it makes it a better polishing stone than the yellow.
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Here's a quick jigane comparison I did recently off two sides of the same stone. These deeper scratches on the yellow side haven't come from having finer abrasive (obviously), or more abrasive, it's come from
pointier, sharper abrasive. And I reckon that's what makes them quicker.
Yellow:
Blue:
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Anyhoo... that was all just speculation really! Here's another little look at the Maz polish from my original post. I think BBW is a pretty classy kasumi stone

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