The eye's limbal ring goes in the category of overlooked but not unseen. It's the dark circle around the iris that enhances the whiteness and brightness of the sclera (the whites of the eyes). Researchers at the University of California at Irvine asked people to rate sets of faces that were identical except for the eyes—one had dark and distinct limbal rings and the other had none. The outcome? Whether male or female, the faces with prominent limbal rings were considered more attractive.
The likely reason is that a dark, well-defined ring is a signal of youth and health—qualities that people seek in mates. It's thickest and most prominent through the early 20s and fades—often becoming nonexistent—with age and medical conditions.