Jewellery's ancient Child: Signet Ring

Past Masters ring
From the series of Masonic signet rings By Adone T. Pozzobon
18k Gold

http://www.engravingarts.com/pgGal5_PM-Masonic1.html

Napoleaon's signet ring
For the Commander of his Honor Gaurd, Hans de Bruyere of Nimegue.
10.5 ct of diamonds
Gold

http://www.napoleonexhibit.com/item.php?id=69 

A signet ring or signature ring is a ring that has, in modern times, been characterised by personal engravements to create the seal/signature. Otherwise, it was originally used by authoritative figures (Egyptian Kings, popes, politicians etc.) to seal important documents. The seal served as prove for authenticity of document i.e. there was one owner of seal and if tempered with seal cannot be forged. The seal/signature is traditionally the top of the ring. It can be metal, gemstone or semiprecious materials.

 https://en.wikipedia.org/wiki/Seal_(emblem) 

http://www.wisegeek.org/what-are-signet-rings.htm