I have a black line under my fingernail in my left thumb. What is that?

I heard that having black lines means it causes heart problems. i talked to my dermatologist and he said that he thinks its a nail mole. Please respond asap.I have a black line under my fingernail in my left thumb. What is that?
It's just your thumb, don't worry about disease or syndromes or vitamin/mineral deficiency. Are you a darker skinned person? That would be entirely normal. On the other hand-





Melanonychia (also referred to as melanonychia striata) is brown or black pigmentation of the nail plate. It commonly takes 2 forms: diffuse melanonychia or longitudinal melanonychia (LM). The most important cause of melanonychia is subungual melanoma (SM), although other causes include physiologic LM, systemic causes, trauma, inflammatory disorders, fungal infections, drugs, and benign melanocytic hyperplasias (eg, nevi, lentigo).





and it looks like the picture on the right:http://www.mmt-fr.org/IMG/jpg/naevus-mat鈥?/a>





ALWAYS vertical and whole nail. So if it doesn't look like that, think little trauma to vessel under nailplate....I have a black line under my fingernail in my left thumb. What is that?
I had that problem a while ago and it was b/c i smashed my toe, for me it was just dried up blood. At first i thought it wouldn't go away but it did as the nail grew.
First two things that come to mind are either dirt, or blood coming up from under the nail (caused from hitting the nail making it bleed underneath).

this works