Agree that tiffany, and especially pandora is as far as i was aware seen as kind of tacky and 'common' for want of much better wording., depending what it is. I think it doesn't help that as they've been churning out the same designs for years, it is hardly fashion forward. I know some people will argue it is classic but i don't think that is usually what teens care about. Their more high end stuff might be nice but the accessibly priced jewellery is pretty ubiquitous . How does she dress? if she dresses in an edgy or cool way, i'd say maybe not to be honest. If she's quite girly and classic, then perhaps you are on the money after all?
I really don't agree at all that those of us saying it is naff and has certain connotations, differing tastes don't always need to be about class, what's on trend in one part of the country isn't going to be in another at the end of the day. Not sure that makes anyone a 'chav' - which is an awful word. All brands have demographics and connotations at the end of the day, that is how branding works.
Could you not ask her if she would like it directly OP? the thought behind it is lovely but its even lovelier if she agrees herself, and wants to wear it. In however many years she will be looking back to the lovely necklace she wore all the time at 18 that you got her if its something she likes, otherwise it will realistically be a case of faux enthusiasm for the gesture and it going in a box. I often find the jewellery with meaning is the jewellery i love and wore.
I'm sure she would also bloody love a trip to NYC for her 18th? she could pick something there, and that really would be special!
Happy to help if you fancy suggestions for alternatives, feel free to PM! Ignore the comments about the baby bracelet, honestly i don't really know what it has to do with anything, certainly no need to criticise it