For some reason my first thought was Lara Pulver, but she's in her early 30s even in Sherlock and there really aren't a lot of icons out there. She might be a little sharp and knowing for what you want anyway, even her smile has edges.
There's a great Kaya Scodelario set here where she looks much more grown up.
Adelaide Kane might work if you don't care about eye color, and there's plenty of other sets, of course.
Candice Patton and Aja Naomi King are both gorgeous classy black ladies with beautifully styled hair, which seems like it would be important for Rarity whatever her race.
And then I stumbled on GracePhipps, who would probably be my choice, so I figure I'll stop there and see what you think. :)