Jewelry, what is this ring?

I have a ring that i have inherited from my grandmother. I have no clue the origin of the ring and whether it is real or not. The metal of the ring is a white-silver colouring. I wore the ring and it has not turned my finger green either so i do not think it is br but i am not sure if it is silver. The stone appears to be a "diamond" ;however, it looks like there is scratches inside the "diamond" and the facet is not waxy, it is clear.and smooth. The "diamond" is held real tight by prongs. The ring has been laying in her jewel box for 22 years and has not tarnished at all. When i tried researching the hallmark, nothing came up. Has any one ever seen a hallmark that looks like some like this -> <-U-< 5 ? the arrows have a line straight through connecting? Please help!