What’s so special about the “sampietrini”, those little square bricks typical of Rome’s old roads? The sampietrini have been used since the times of pope Sixtus V in the late 1500s but the name was invented later in 1725 after the new paving project of St. Peter’s square. “Hard as a sampietrino” is a common expression in Rome. Sampietrini are made of a hard and heavy volcanic rock. It is very interesting to a geologist: at first sight, you might confuse it with basalt; well, maybe a special kind of basalt. Actually, it is not correct to call it basalt. It is a rather rare rock around the world, although not in the Mediterranean area. But everything would be clearer if we knew more about the basalts themselves…