For extra-special occasions, skip the cocktail jewelry and go with the real thing. Fine jewelry can add class to an outfit; whereas cocktail jewelry can downplay the look you are trying to achieve.
Fine jewelry does not have to be expensive; instead focus on less-expensive jewels that are quality-made.
Stones should have the right style and cut; they should fit in their settings accordingly. Consult with a jeweler on what you are looking for, what your outfit looks like and what kind of style suits you.