Indeed this is an interesting case, from my research it looks like perhaps the best way to represent this is the following:

surface=paving_stones
paving_stones:pattern=rectangle
paving_stones:material=granite

I went ahead and added those tags. Feel free to reopen this note if anyone disagrees with me! And thanks for your contribution.

Sources:
https://wiki.openstreetmap.org/wiki/Tag:surface%3Dpaving_stones#Subtags
https://community.openstreetmap.org/t/type-of-stone-in-surface-surface-granite-and-similar/9424/4