Another type of Medieval rings - carved cameo. They became popular again in XII century, and were used as a signet rings. Remarkable fact is that first cameos, used in these rings, were mostly carved during the Roman period and found in former cultural or trade centers. 
In XIII century stone carving was revived in Holy Roman Empire.

Ring framed cameos were oftenly surrounded by inscription - owners name or latin phrase, so this shows that cameo rings were used as a seal for a personal letters, or something, that contains confidential information.