What would cause a seemingly healthy 30 to 40 year old hispanic woman to have gray finger nails?

I am in a medical class...and though class is out I am curious as to why someone would have gray finger nails. I first assumed diabetes but I need to know and I cannot find it anywhere on the internet. Please Help. Thank you.What would cause a seemingly healthy 30 to 40 year old hispanic woman to have gray finger nails?
Maybe this will help:


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