You may have splinter hemorrhages if you have dark, vertical lines under your nails that look like splinters. The most common cause is trauma to the nail. Splinter hemorrhages can also happen due ...

Medically reviewed by Susan Bard, MD A black line on the nail could be normal, such as when it is just a color variation.

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High cholesterol can result in unusually thick nails or brittle ones that break easily. In rare cases, dark lines, known as splinter hemorrhages, may form under the finger or toenails. The NHS ...