The term "statement" is perhaps overused right now, but it's there for a reason. Statement jewelry speaks for itself. It says "bold," "high-fashion," "earthy" or "glamorous," and it needs very little in the way of support.
In fact, supporting accessories can detract from it. A statement piece, whether earrings, a necklace, a bracelet or a magnificent cocktail ring, does best as the center of attention, and it's this quality that makes it essential. It can say so much on its own, you needn't put a ton of effort into the rest of your look. On a night when you're running late and have somehow ended up in head-to-toe black, wear a primary-colored, tribal bib necklace or cobalt-blue, shoulder-grazing chandelier earrings for all the pop and interest you need to turn monotony into something simply, utterly stylish.
If you must choose one piece of statement jewelry, consider a ring, which is unaffected by neckline, hair style or sleeve length.
Next, one with purpose!