By chance, as we call it in Middle Earth, the BBC Web site just happened to put up an article about a ring found in Ireland: Gold 'posy' ring found in County Antrim field. It is a ring with "the Old English inscription 'I noght on gift bot gifer' ... 'Look not on the gift, but the giver'."

Also known as a betrothal ring, it pre-dates the custom of proposing with an engagement ring, but essentially served the same purpose - men and women exchanged them from the 1500s onwards to symbolise their future commitment to each other. ... Elise Taylor, curator of applied art at National Museums Northern Ireland, told the court that it was tradition to have an inscription on the rings. She said that the name posy related to the French word for poem - poesy.

By "engagement ring", maybe they mean specifically a ring with a diamond or other gem?

The Catholic Encyclopedia of circa 1910 has an article on Betrothal. The last three paragraphs touch on betrothal rings from the late Roman empire on, though not with much detail. BTW, the section says that (in their terminology) "betrothal" is stronger than "engagement"; the distinction is described earlier in the article as the difference between a contract to marry (binding, formal, enforceable in church and civil law, an impediment to marriages) versus a mere offer of future marriage.

I'm afraid that's all I know. Maybe a Web search for "engagement ring", "betrothal ring", "gimmal ring", or "posie ring"?