Tiny black lines on finger nails may be caused by subnail stasis, nail malnutrition and onychomycosis. 1. Subnail stasis: When the nail is traumatized or squeezed, it can lead to the rupture of capillaries in the nail bed, resulting in subnail stasis. 2. Nail malnutrition: If the daily diet lacks protein, vitamins and trace elements, it will lead to nail malnutrition, which will lead to melanin precipitation and the appearance of black lines under the nails. 3. Nail nevus: the main symptom is black lines on the nail, which grows with the nail without itching and pain. If it grows slowly, no special treatment is needed; however, if it grows rapidly and shows symptoms such as pain, it may be malignant and requires timely intervention and treatment. If you find unexplained black lines on your nails and it affects your quality of life, go to the hospital if necessary to avoid delay in diagnosis and treatment.