All of my very healthy fingernails, except two, have the semicircle of white you describe. I think this is normal, but your question has piqued my interest. I think I'll google stuff and see what I find.
Hello,
White marks that you are describing can be leuconychia. There are several possible causes for the discoloration, but the most common are mild trauma, a fungal infection, skin conditions(like psoriasis), or nutritional deficiencies(zinc and calcium).Take some nutritional supplements containing zinc and calcium for some days and see if the symptoms improve.If they persist then pls get it examined from a dermatologist and get fungal infections ruled out.
Hope it helps.Take care and regards.