The geology of Rome’s sampietrini

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, but not in the Mediterranean area. But everything would be clearer if we knew more about the basalts themselves…

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