Heterochromia isn't just a matter of genetics or chance: it's a small masterpiece of nature, a spark of difference that brightens the eyes and makes each person even more unique.

For example, one eye may be green, and the other may be brown, blue, or another color. Segmental heterochromia: This type of heterochromia is similar to central heterochromia.

People with heterochromia—or different-colored eyes—have a striking and unique appearance. Here's what causes it and just how rare it is.