Back to the fingernail conversation… Look at this page about fingernails http://www.normanallan.com/Med/askdr/finger.html
I have ridges and have always heard it means malabsorption or could be dry nail beds lacking oil due to chemicals or hard detergents. It can also be hereditary. But this page says it could also be kidney or low iron.
“Lengthwise grooves or ridges may indicate a kidney disorder and is associated with aging. An iron deficiency may also cause ridges.”