What Causes Dark Band Around The Fingernails?

I have just noticed that all of my fingernails have a darker band under the nail, starting where the nail separates from the finger tip. It runs from side to side and is about 1/8" or less in depth. This is new in the past day or so, and I have never seen this before. I am 70 years old and a non-smoker. Should I be concerned? The has been no change in my diet or lifestyle that might explain this. The only medication I take, and have for years, is 20mg of Crestor daily.