The gift that gets worn is almost never the loudest one in the box. It is the one that fits into what she already does every morning without a second thought.
Start with what is on her hands and neck right now. The metal she has worn for thirty years is the metal to buy. A chain that sits at the collarbone works with everything she owns. Small earrings she can forget she has on will outlast any statement piece.
What if I don't know her taste?
Look, don't ask. The jewelry she wears on an ordinary weekday tells you the metal, the scale, and how much she likes to be noticed. Match all three and you cannot go far wrong. If you truly can't check, a simple pendant on a 16 to 18 inch chain is the safest shape there is — it fits every neckline and every age.
Should I add an engraving or a message?
A message, yes. Engraving, only if the words are specific to the two of you. Every order here can carry a private note she opens when the box arrives, and a line she recognizes is worth more than any inscription.