What causes black lines under fingernails?

Black lines under fingernails can be caused by various factors, including splinter hemorrhages, which are tiny blood vessels that break and bleed under the nail, often due to trauma or certain medical conditions. They can also indicate more serious issues, such as melanoma, a type of skin cancer, particularly if the lines are pigmented and changing. Other potential causes include fungal infections or systemic conditions. If you notice persistent or concerning changes, it's important to consult a healthcare professional for evaluation.