Jewelry Boutique

My girlfriend knows it's coming. But she doesn't know when or how. I think she expects a diamond ring.I want to surprise her with an emerald engagement ring. Reason? We both have green eyes which is rare and really cool (were both of Mexican descent). Three guidelines.
First: I would like a Platinum band. (This way it can grow with us EG: Maybe add diamonds later)
Second: A Celtic knot or sometimes called Eternity knot would be cool to have
Third: Under $2000-$2500 please. Somewhere in the $1200 to $1800 range would be great.
I live in the Los Angeles area if that helps.
I look forward to your suggestions and thank you all in advance.