it looks like a c. 30s style engagement ring. the white metal is probably white gold, but could be platinum. using white metal makes the stones appear larger. i wouldn't say it's a top of the line ring, and makers of these sometimes continued using older styles of cuts into the 20s and 30s, even after more advance round brilliants were developed (it could also be an antique heirloom stone that was reset into a new ring).
it would most like be an old european, not old mine. can you see the bottom of the center stone, is pointed or does it have a small flat surface?
in the second picture you post, the band looks really thin--before giving it to your girlfriend i would be sure that it is structurally sound and won't crack or bend.