The common causes of black spots in nails include trauma, nail dystrophy, onychomatous nevus, melanoma, fungal infections and so on. 1. Trauma: If the nails are subjected to external pressure, collision, etc., it is easy to form subnail bruising, which may manifest as black spot-like appearance. 2. Nail malnutrition: if the patient lacks vitamins or anemia, it will easily lead to nail malnutrition, which will form the appearance of black spots under the nails, and this black spots can gradually become longer with the growth of nails. 3. Nail nevus: a kind of benign tumor, which can also be manifested as black spots inside the nails, and usually the black spots will be connected to the root of the nails. 4. Melanoma: it is a malignant transformation of melanocytes, which can also appear in the nails, manifested as black dots in the nails, and it is recommended to carry out dermatoscopy in time to confirm the diagnosis and treatment as soon as possible. 5. Fungal infection: commonly known as gray nails, is caused by dermatophytes invading the nail plate or nail bed. In addition to causing the manifestation of black spots in the nails, it can also make the surface of the nails uneven, discoloration, peeling and so on. If there are black spots in the nails, it is recommended to go to a regular hospital in time to clarify the cause of the disease and carry out targeted treatment.