The piece a daughter keeps is the one attached to a moment she can name. Price has almost nothing to do with it.
For a teenager, choose something she can wear every day without worrying about it — small studs, a thin chain, nothing that commits her to a look she is still deciding on. In her twenties, one good everyday piece beats three adequate ones; she is building a small rotation, not a collection.
What is the safest choice if I know nothing?
A short chain with a small pendant, in the metal she already wears. It fits everyone from twelve upward, works with every neckline, and does not tell her who to be. It is the least interesting answer and the one that gets worn.
Is jewelry a strange gift for a milestone like graduation?
No. A dated event is exactly what makes a piece worth keeping — it carries the day with it. If you add words, add the year rather than the achievement; the year ages well.