Premodern medicine used a variety of mineral substances for therapeutic purposes. The present article deals with pitch-asphalt, and, in particular, a precious kind of it called mūmiyāʾ originating in Persia. It was first described in detail in the Arabic pharmacological tradition, and its fame spread throughout the medieval Mediterranean, including Byzantium. By editing and examining for the first time a previously unexplored medieval Greek text on mūmiyāʾ, this study offers new insights into the medicinal uses of this substance. It also significantly increases our understanding of the intense cross-cultural transfer of medical knowledge from the Islamicate world to Byzantium by showing that this was not merely based on the translation of a few Arabic medical works into Greek, but was a multifaceted phenomenon involving a complex nexus of sources that require further investigation.