@StraightRazorDave:
Haven't honed a razor on it yet. My razor honing skills are limited; it rather differs from honing knives and tools. I can manage, but would hesitate to give definate descriptions of hone behaviour - that's best left to the likes of JimmyHAD, Hi_bud_gl, Bart, ... .
The two sides feel the same. I presume the differences in colouring due to having been set in a box, combined with a scratch all around (from scraping grime from the box?) gave that impression. O, and the bottom had a more even and smooth layer of grime, whereas this was scratched and not as thick on the top.

@ChrisL:
It is definitely sandstone-like. Silky maybe wasn't well chosen; velvety might be more like it. I didn't mean to convey the idea of smooth 'with a very even and dense surface, frictionless', but more 'kind of soft to the touch, with a bit of draw'. I can't quite describe it - hour glass sand glued together with candle wax maybe?
If I'm not mistaken, there's a thread somewhere about DY, discussing their differences in make-up; all rather coarse in effect, but some apparently shedding marked quantities of sand-like particles, and others not displaying this behaviour at all. This one doesn't - could tactile characteristics vary likewise?

Mind however that my denomination of a DY is a guess only - an educated guess maybe, but a guess no less. I've seen DY's before (pictures and real life) and read about them, but never used one. I might very well be mistaken!