ok i will give you the answer to be fairly and honestly. my mama is a well reputed doctor so once she told me about this what is the cause of dark stripe on the finger nails. there could be many causes
1. splinter haemorrhage what is splinter haemorrhage? so basically it occurs when blood vessels under the nail finger damaged.and it happens usually we our nail get injured or by hitting somewhere with hard thing
2. subungual melanoma it is a kind of pigmentation which could be spread with the passage of time. it can be hurt and bleed as well so at initial stage it could be single black line and i told before it would be spread and hurt so never ignore such kinda stripe so tell me what exactly kind of stripe you have so i can better help you :)
Most commonly, black-striped nails are caused by symptoms known as linear melanonychia. According to the United Kingdom's National Health Service, these lines usually occur in African-American descendants aged 20 and older.
Linear Melanonychia is considered a normal nail color variation. This condition occurs when pigments in the nail known as melanocytes make excess pigments. This causes black nails.
Less common causes of melanonychia nails include:
Use certain drugs, such as chemotherapy drugs, beta blockers, anti-malaria drugs, or azidothymidine drugs
Another cause of black-striped nails is bleeding, which occurs when blood vessels under the nail are damaged. This condition is usually due to injuries, such as being hit by a blunt object or because of a hit.
Even more serious, black-striped nail marks can indicate melanoma, a dangerous form of skin cancer. Melanoma under the nail is known as subungual melanoma. One type of melanoma is known as acral lentiginous melanoma (ALM).