Cobblestones

Many of the roads in old Rome are paved with black cobblestones, called sampietrini and they are, for the most part, a specific kind of porphyry, an igneous rock.  They come in different sizes and Rome is famous for them. These large ones are on the footpath in front of the Arco di Constantino, adjacent to the Colosseum.  Most of the stones used on the heavily traversed urban roads in central Rome are actually smaller.  

Here is a short, interesting article about the sampietrini.  It turns out they were invented and adapted as a paving material in the mid-1500's, under the authority of Pope Sixtus V, who looks very grumpy in this picture. (Frankly, most of the papal portraits we saw are pretty stern...it must be grim work.)

As it turns out, the current municipal government of Rome is beginning to replace it's cobblestones with that newfangled asphalt, which is making lots of other folks unhappy, too.