I have a vintage yellow one like that, the back looks like it was hand cut or chiseled. It's a *fantastic* stone. It's a touch faster than my very clear (or at least more closer to clear) translucent, but still makes at least as fine or maybe even a finer edge. I figured the yellow one would be just a translucent that'd be colored by age and oil (and maybe it is), but it is totally non-porous (so it's not a lesser grade), but still does not pass light through the middle with a flashlight.
I haven't had a chance to finish a razor on it yet, though.