There seem to be only a few English words that begin with the letters P-O-R-P. Next to porpoise, it's basically just porphyry, "a hard Egyptian rock having red and white feldspar crystals embedded in a fine-grained, dark-red or purplish goundmass." (Porphyry can also refer to a philosopher, a bishop, a vineyard, an island, and a system for dividing the horoscope!) Porphyry is Greek for "purple," the traditional color of royalty. "Imperial Porphyry" was a deep purple igneous rock with large crystals of plagioclase and was used in the building of monuments and other structures in Imperial Rome and beyond, for example in Hagia Sophia in Istanbul and in the "Porphyra," the official delivery room for pregnant empresses in the Great Palace of Constantinople. Napoleon's remains are buried in a porphyry sarcophagus as well. The word now refers more generally to any igneous rock with similar properties. There were eight cases of Porpert* in OhioLINK and in 337 in WorldCat.
(Ancient Roman bathtub in Egyptian red porphyry, transformed into the baptismal font of the cathedral in Milan, Italy, from Wikimedia Commons.)