Omphalolith

An omphalolith, also known as a umbolith, omphalith, navel stone, or umbilical concretion is a periumbilical mass that may form due to the accumulation of sebum and keratin. The colour is black or brown, and may be related to the skin type of the patient. It may resemble a malignant melanoma. It may be caused by poor hygiene, and may form in retracted navels in obese people.

Etymology
The name was derived from the Greek words omphalos (ομφαλός), meaning navel, and lithos (λίθος), meaning stone.