The type is based on the figure of Ares usually attributed to the sculptor Alkamenes, which originally stood in his temple in the Athenian Agora. It is only known from Roman copies, the best of which is the "Ares Borghese" now in the Louvre, fig. 104 in Bieber, Ancient Copies. The prototype was favored during the Imperial period for depictions of the emperor, sometimes as a group with the empress appearing as the god's consort Venus.
Bigio morato is a fine-grained gray marble, much favored during the 2nd century A.D., especially during the reign of the emperor Hadrian, whose villa at Tivoli has yielded a number of fine sculptures. For a recent discussion of this rare material see Cioffarelli, p. 67 in Anderson and Nista, eds., Radiance in Stone, Sculptures in Colored Marble from the Museo Nazionale Romano.