There are different causes for and types of dark circles. Hereditary dark circles present in some families are genetic and are caused by very thin subcutaneous tissue that allows the muscle underneath to show through, adopting a characteristic purple colour that is very difficult to cover up and that often becomes more noticeable with age. These dark circles are also often associated to sunken eyes (deep nasojugal folds), which means that the eyelid casts a shadow and makes them more noticeable.
Racial dark circles are a common feature of darker skins, such as Arabic skins, and are also of genetic origin. In this case, they are caused by accumulated pigment in the skin.
Lastly, there are acquired dark circles that are caused by pigmentation alterations, such as spots in this area caused by the sun.