This is the parvis of the Basilica. Parvis means 'square in front of the church. '
In 1757 Honoré III of Monaco used white and black pebbles to create the mosaic which represents weaponry and the Grimaldi's coat of arms. The curved 'H' signifies Honoré III. Menton, of course, was ruled by the Princes of Monaco until the French revolution.
After centuries of feet, the tops of some of the pebbles are worn totally flat. The photo below was taken very near the entrance to the church.
On the other hand...a few years ago we had massive flooding in Menton and the rampes (the steps leading up to the Parvis) and also perhaps the parvis itself, had to be restored. The flooding had lifted many of the pebbles. So perhaps the reason the tops of some pebbles are perfectly flat isn't the effect of centuries of wear but of fairly recent renovation and brand new cut stones. I prefer to think it was caused by feet over the centuries!