Hi. have seen some very light black vertical lines on my finger and toe nails- what could this be due to?

Variety of reasons : Hi, those lines could be from simple small bleeding from damages to tiny blood vessels under the nails from getting hit etc or vitamins/nutrition deficiency, and all they way to a more serious skin disorder. Most of the time it’s simple/benign. See your doctor who can evaluate your history/medical info, examine your nails, and give you personalized treatment if needed.