studying the way to modify the vertex groups for use in other modifiers after the geometry nodes, I “found” this way to get a cobblestone floor; the result is a bit heavy, but maybe it can be useful to someone.
It almost looks like some kind of “modern art.”
This looks great! 
With a material shader pixelating (pebble-ating?) other still images & videos, you could do mosaics & mosaic-animation with this. …a short video on the history of Byzantine iconography, or daily life in Pompei would be super-awesome in this style!! … or even another take on “Bad Apple” 
I think there are already plugins for making mosaics, but it would be nice to know how to orient the pebbles based on the shapes of the image, like in real mosaics, but I don’t know how to do this, for now, it will be the subject of other experiments… 
I think that you can console yourself with “plenty of ‘artistic license’” in this case. Whether or not “an actual mosaic artist” might have come up with this particular degree of internal symmetry on his own, it “looks good.” And that might well be enough.
it might be enough also because it wasn’t what I was looking for: I was actually trying how to modify a vertex group in geometry nodes to use it in another modifier, but in this case it’s not even needed… 
