Questions about fingernails - black lines

[Dakini]

Hi,
I can't find any answers on a google search, and plenty of people have asked.

What is the cause of short, hair-like, verticle black lines in the nail beds?
Would be located on the thumbs and small pinkie toes, for some reasons. Not due to nail trauma.

Is this heavy metal? I have noticed that they have been improved, so far, with MMS.

Any input?

Thanks!