What causes dark lines under fingernails?

Injury: Trauma and some illnesses can do this. Remember that nails are related to hair but grow more slowly so that an injury to them may show up over time. I often see this in my oncology patient with certain chemotherapy. They well have lines in the nails.

Different causes: Black stripes down 1 nail may be subungual hematoma. Red or brown streaks may be splinter hemorrhages. Brown/black lines from nail plate to tip are often seen in people with dark skin & are normal. Subungual Melanoma can cause nail ridges & dark colors of cuticle. It most often effects the thumbs or big toes. Dark lines may be due to other illnesses. Get checked out when in doubt.