The Mediterranean basin is characterized by a remarkable variety of widespread gypsum outcrops ranging in age from the Permian to the Holocene. Among them, the Miocene deposits contain large gypsum crystals filling fractures and ancient karst caves that were exploited by the Romans to produce thin transparent cleavage plates to be used as substitute of glass in window panels (lapis specularis). For their size (at least some decimeters across) and perfect transparency, the most sought crystals were those from Spain, which is also the area with the richest lapis specularis deposits. In Italy traces of mining activity have been discovered only recently, while in Cyprus and Turkey, the quarries described by the Roman sources are still unknown. The increasing number of excavation sites continuously discovered in the Vena del Gesso of the Northern Apennines suggests that important discoveries could be made in Sicily, Cyprus and Turkey through specific studies. As indicated by our geological analysis, lapis specularis crystals could also be present in Albania, Algeria, Egypt, Jordan, Greece, Israel, Romania and Tunisia. These locations could represent potential sites of excavation of Roman not mentioned in ancient sources.