Never buy a stone/ring combo at a jewelry shop or Spence type place. They make it easy, but you end up pay too much for the stone you get.
Do some work and buy the stone separately. I highly recommend
Blue Nile. You can run all types of filters (price, carat, color, cut, clarity) and pick the exact stone you want.
Then either put it on a ring of your choosing, or put it on a super plain silver band and let her design the ring.
It can be really tough to get standard jewelry store to just sell you the ring and have them put your stone in it though...which is BS. That's when I realized how much they just want to rip you off on the stone.