A diamond and some gold.
It can be kept until she wants something made out of it. Obviously other stones are available, maybe her birthstone. Maybe a promise to have something made for her 21st birthday.
Coins, the royal mint has just issued 50p with the King on, a brilliant uncirculated is £11.
If you want to spend more there are bars of gold and sovereigns, 1/2 sovereigns and 1/4 sovereigns. I got some years ago for my nephew and nieces, they are nice to loo at and if they are ever broke they can sell them.
'Polaroid' camera, I've got one and it is a hit with teens at family gatherings.
If she likes reading than do you think she would like to learn about the linguistics of the English language. David Crystal's books are beautiful and are better in book form than kindle, the Cambridge Encyclopedia of the English Language is the kind of book you dip in and out of, it talks about the evolution of the English language. Why we spell things certain ways and changes in vowel sounds.
www.cambridge.org/highereducation/books/the-cambridge-encyclopedia-of-the-english-language/2B10AC8766B73D09955C899572C1E7EC#overview
I recently got this for a friend.
www.amazon.co.uk/dp/4805314680/ref=pe_27063361_487055811_TE_dp_1
He's actually learning Japanese but it is a beautiful book, it has stories in English on one page and Japanese on the facing page and note about the grammar and translating.
I'd say get it as a book, it is available on kindle but the book format, IMHO works better.