Can you identify this US Hallmark?

by Dawn
(Nevada)

I originally thought this ring was 18 Kt, but after looking closer to the Mark it looks like 18B then 2 o's underscored with a raised dot above the o's. There are also initals engraved G to L, so I believe this was my 2nd great Grandmothers who was married in 1864 in Indiana. Photo Attached. Thank you for your help.