What Do Black Stripes On The Finger Nails Indicate?

For the past several months I ve noticed my pointer finger on my right hand I am right handed keeps developing a black stripe done it, from cuticle to tip of nail. I have gently filed it down, it s still there. I have come to the conclusion that this may be an issue. It s just the one finger. I have checked all digits carefully, this is the only finger that has developed this characteristic. Any insight would be much appreciated. Warmly, Bonnie I am 60 years old, very rarely ill. I have been a smoker for 40+ years, although I have cut way back I do still smoke. I was a heavy drinker (at least 3 drinks most days) for the past at least 7 years.