Detail of the San Vitale Abel and Melchizedech Mosaic: The Sacrifices of Abel and Melchizedech
Basilica of San Vitale, Ravenna, Italy
This is the main lunette of the mosaic. On the left Abel offers his lamb, while on the right Melchizedech offers bread and wine to God, who is represented by the hand above the altar.
As in a similar mosaic in nearby San Apollinare in Classe, Abel wears a tunica pellicea, a tunic of skin or leather, like the ones that God made for Abel's parents. But the red cloth covering this reminder of the Fall hangs like the priestly chasuble such as seen on images of Ravenna's bishops (example). Abel stands before a simple shrine of wood and reeds; Melchizedech, in priestly garments before an elaborate basilical structure. Melchizedech's mantle and boots resemble those of Caiaphas in the Passion mosaics of Sant'Apollinare Nuovo, which are also from the 6th century and in Ravenna.
For Abel's sacrifice, see Genesis 4:1-10. Abel is traditionally taken to be a type of Christ as victim and priest, the one because of his slaying by Cain and the other because of the lamb he offered. Melchizedech, whose sacrifice of bread and wine is in Genesis 14:18-20, is explained as a type of Christ in Hebrews 7.
This image in full resolution
The mosaic on the facing wall, with Abraham's sacrifice and his Three Visitors
More of Abel
More of Melchizedech
Photographed at the basilica by Richard Stracke, shared under Attribution-NonCommercial-ShareAlike license.
Sant'Apollinare in Classe has this mosaic of Ursicinus, Bishop of Ravenna (533-536), wearing a white pallium over a folded chasuble over a white dalmatic. This is the typical garb in which Ravenna's bishops were pictured.