Engraved or incised work where the design is sunk into the surface is called intaglio, and is the opposite of cameo, where the design is in relief. Intaglio designs are made on gemstones, glass, ceramics, stone, or similar material. It is the most ancient form of gem engraving: the earliest known Babylonian cylinder seals date from c. 4000 bce. 
This antique intaglio is carved into carnelian and set in a simple yet beautiful rolled gold setting. Gold gilding was added to accentuate the profile of what looks to be Mercury. Great condition for its age!