This monograph looks at the stone material par excellence of the Roman Empire, the most expensive in the Edict of Diocletian - Red Egyptian Porphyry. Starting from the study of this marble, the author traces the history, art, architecture and historiography that over the course of two millennia have recounted the political, artistic, religious and symbolic values of Porphyry. The colour of porphyry represented royalty in the time of the Lagids, the authority of the Roman Empire and the power of the Catholic Church - it was a fundamental colour in the Cosmati middles ages, highly sought after by the grand dukes of Tuscany and the richest element of Baroque opulence and the neoclassical renaissance. The strength of this study, in addition to its historical and photographic components, lies in the corpus of almost 1,000 profiles on porphyry objects, more than half of which are previously unpublished.