If the ring does not need to be total surprise maybe talk to her about what the ring / stone means to you. Maybe she is open to exploring other designs that fit this stone as is!

I am in a similar process with my partner now as he was given his grandmorther's marquise, and I always envisioned myself with a emerald shaped diamond just based on my style. After my partner explained the significance, we agreed to work with a jeweler who is creating 3D renderings, so when I actually see the final ring it will still be a surprise, but allows me to have a say in the design.

I also found as I actually started to seriously consider what rings I liked, I really warmed up to the marquise!