Thanks all for your input. She already has the open heart pendant. Actually that was one of the first things my husband gave me while we were going out and I lost it in Cairo at a hotel Suffice to say, I never got it back.
I really like diamonds by the yard but now im beginning to think i shouldnt give that to her cuz shes only 17, i want to get her something thats more wearable, so now im leaning towards either 2 or 4 but feels like 4 is a strong statement since its a key with tiffanys signature color in it so i guess ill think about it.