What causes black marks on nails?

Q: Some of my finger nails have got small, identical vertical black marks (small lines), slightly below the tips. I have heard in a radio program once that this precise condition could be an indicator of some particular condition/illness, but dont remember exactly. Can you please let me know what this indicates? Also, the nails of both my big toes had broken off at some point and have regrown. But they get very dirty since there seems to be a huge gap between the skin and the nails. Is there any solution for this?

A:Probably, it is malanonychia striata. For your toe nail becoming dirty and friable, fungal infection should be ruled out. Please consult a dermatologist.