Costume Ring with Green "Stone"

I found this ring recently I would love get some information. It is unmarked, which makes it for me very difficult to date it. I am pretty sure it is a costume piece, maybe gold plated. It looks pretty convincing but I am certain the "stone" is glass. There is some wear on the ring but no other metal shines through. The green glass is in perfect condition - no scratches or other wear at all. I think that for a custom piece or an imitation item it's well made.

I bought it because I love the green colour and because it is similar in style to a ring my grandmother bought me in the mid-seventies. The ring she bought me is 10K gold.

Does anyone have any idea about the age or the maker of this kind of jewelry?