Hi blot-sven,
Your ring seems to
Byzantine, as it has a typical
Byzantine Greek invocational
inscription. The impression of the ring seems to read +KER / OH(Theta)H IW / ANNH (?), 'Lord, aid John' (Kyrie boèthè Ioannè), the word KYPIE is contracted KE, and beta has the form of R,
as is conventional from about the 9th century until the 13th century, but I think your ring would date to the 9th-10th centuries.
Regards
Gert