Black lines like thorns in the nails may be caused by trauma, onychomycosis and other diseases. 1. Trauma: If the nail area is bumped or squeezed, the local soft tissue is damaged and bleeds, which will cause the accumulation of bruises or black nail color and black lines, and some patients will also have swelling and pain and other uncomfortable symptoms. 2. Mole of the nail: Mole of the nail is a kind of mole that grows under the nail plate and belongs to a kind of mole. The common symptom is nail black line, which can make the patient’s nails have black lines like thorns. There are many reasons for its formation, the common ones are acanthosis nigricans, Addison’s disease and vitamin B12 deficiency, etc. The local triggering factors are trauma, fungal infection, eczema-like changes, and bleeding under the nail. Most of the black lines like thorns in the nails are caused by trauma, onychomycosis, etc. Generally, there is no need to be overly nervous. If uncomfortable symptoms occur, it is recommended that patients seek medical attention for relevant examinations to clarify the diagnosis.