This unusual ring is carved from a single piece of chalcedony, slightly oval in form, round in section, expanding somewhat towards the bezel. Deeply engraved on the bezel is a frontal bust of Jupiter Ammon, the device framed by a raised border. The god is shown with a full beard and mustache, long wavy hair, and the underslung horns of Ammon, with drapery flowing over both shoulders.
Jupiter Ammon, or when a modius is present, Serapis Ammon, was one of the most popular subjects on gems beginning in the Hellenistic period and continuing into the Roman. The god can be shown as a full figure, bust or head, usually in profile but also frontally, as here. For a recent study see R. Veymiers, Sérapis sur les gemmes et les bijoux antiques, especially nos. I.AA1-90 for frontal busts and VI.BA1 ff. for Serapis Ammon.
Rings carved from amber or semiprecious stone were popular during the Imperial period, plain or with elaborate hoops, their bezels either in relief, in intaglio, or without ornament (for the type see E. Gagetti, “Anelli di età romana in ambra e in pietra dura," in G. Sena Chiesa, ed., Arte e materia. Studi su oggetti di ornamento di età romana. For an elaborate example in rock crystal see no. 46 in J. Spier and J. Ogden, Rings of the Ancient World, and for one of chalcedony of similar form to the present example, see no. 64 in A.B. Chadour and E. Joppien, Schmuck II, Fingerringe.