Nails with black lines and dots can be caused by trauma, nail melanoma, and onychomycosis. 1. Trauma: trauma such as squeezing the fingers may lead to bruising under the nail plate, which in turn may lead to black spots on the nails. 2. Nail melanoma: Nail melanoma is a kind of malignant tumor, which mostly manifests as nail discoloration and longitudinal pigment bands in the nail plate, and some patients may have black lines and black dots on their nails. 3. Nail nevus: It is a benign pigment cell nevus with hyperplasia of pigment cells in the nail matrix area, resulting in the formation of gray, brown or black bands in the nail plate. There are many reasons for black lines and black spots on nails, so it is recommended that the patient should consult a doctor for examination to clarify the cause of the disease.