Basically there are two type of occurrences: hemoglobin degradation & heavy metal.
Our eye area (also called periorbital) is the thinnest fragile skin of face, linked with capillaries, blood vessels so tiny that red blood cells have to stand in line, and bend in half if necessary to get through them. Sometimes it leaks from these capillaries, body begins the mopping-up process by degrading the hemoglobin in the blood, resulting in the dark bluish-red pigmentation/discoloration as visible as panda eye. As you can see, dark under-eye circles are very similar to bruises. The same mechanisms produce them. It is so visible because periorbital area (eye area) is the thinnest part of our face.
What causes hemoglobin degradation?
1) poor blood circulation; medication, stay up late until midnight,lack of sleep, sinus, smoking, liver toxic, long hour computer user，fatigue
2) aging (skin thinner during age)
3) hormonal changes; prenancy or menstruation
4) UV rays exposure cause melanin formation
5) inflammation; allergy, hereditary (genetic)
Heavy metal in pigments of decorative cosmetic is hardly removed by eye makeup remover or any cleanser, low quality cosmetics such as eye shahow, eye liner, or mascara are formulated by heavy metal to contribute sparkling, shining effect when we apply on eye. Heavy metal residues that are not removed properly will accumulate underneath the eye area and form dark eye circle.