Superficial relationship question (for the girls)?

I am now unofficially seeing a girl I have been close friends with for 3 years. I wanted to buy her a necklace for the one month anniversary of our first kiss. the necklace is available in sterling silver, or platinum. her most coveted piece of jewelry is white gold, and though corrupt it may be, I am looking to one-up the prior boy who gave her the ring. My question follows. If I opt for platinum, will she implicitly know it's composition (and not assume silver), and if not is it likely she would investigate it's spotless background? Also, would she hold a Pt necklace in greater veneration, or is the sentiment the same with silver? I am 19, she's 20.