These stones are part of the driveway near the sidewalk. Here's another post showing the uniquely ornamented dwelling. Apparently the whole project began with the concrete.
I seem to be having "Monreale Day" here in Tapirgal's blog world. Please visit two more blogs for more of marvelous Monreale. You can also follow this link to see other posts about Monreale on Animal Art Along the Way.
I believe this was in the Roman section. Since it's on the floor for everyone to walk on, I think it must be a reproduction or a design meant to recall the period.
I also learned from JM that this type of pavement pattern (the black and white wavy lines) originated in Portugal. Having not been to Portugal (yet), I first encountered similar patterns in Brazil. In fact, I posted one of the sidewalks from Sao Paulo during the first week of Tapirgal's Daily Image.