There shoudl be no reason to part with your ring, if you look around you'll find somewhere that can have a look at the ring and the certificate on the spot and you shoudln't have to leave it, as you'll be having it valued for insurnce purposes ( I imagine) I would avoid leaving it behind!
Also, try to find somewhere that just charges a set fee, I paid €75 in Dublin, I believe that if they charge you a percentage of the ring they'r emuch more likely to inflate the value of the ring.
Sorry I don't know anywhere in your area, can offer you the name of the place I went in Dublin if you'll be around!
addob