A black line on your nail (medically known as melanonychia) causes a streak of dark pigmentation that runs along the nail bed ...

These black lines on fingernails may look like bruising or internal coloring - not like you just accidentally created an edgy ...

In addition to onychorrhexis, several other types of lines can develop on nails and indicate different health issues. These can include leukonychia striata, longitudinal melanonychia, longitudinal ...