Look at what she reaches for every morning. That tells you more than any gift guide. If her go-to is a pendant she never takes off, a diamond necklace with a different stone shape or setting gives her a second layer of that same comfort.
Diamond studs work when her ears are bare more often than not. Tennis bracelets sit in the space between everyday and elevated, which makes them easy to wear without thinking about it. Stackable diamond rings invite repetition, one added each year. The best type is the one that fits the life she already has.