Black lines in the nails may be caused by subnail bruising, nevus of the nail and melanoma. 1. Bruising under the nail: If the nail is traumatized or squeezed, the blood vessels under the nail may rupture and cause local bruising, which will lead to the appearance of black lines in the nail. 2. Nail nevus: the formation of nail nevus is mostly related to gene mutation or trauma, etc., and the patient’s nails will have black nail lines. 3. Melanoma: Melanoma occurs on the soles of the feet, under the nails and the ends of the fingers, etc. Its color may change, and it is mostly black, brown or red. The cause of this disease is mostly related to DNA damage, and environmental and genetic factors may trigger the disease. The causes of black lines in the nails are not unique, and patients are advised to go to a regular hospital to clarify the cause of the disease.