bxl-forever
Added surface tag, thanks.

Indeed, OSM has two types of stones.
"surface=cobblestone" is for raw stones, as found in ancient times, but those are completely inexistent nowadays. (I found some in Rouge-Cloître/Rood-Klooster.)
"surface=sett" is for the small stones we see too often in Brussels and other cities.  The ones everyone hates when cycling.
I added another useful tag after your note: "smoothness=horrible".  Some bicycle routing engines use this, so that they can warn users, and some of them will try to avoid taking such a street if they can avoid it.