Why do fingernails have horizontal marks on them?

Diana - Fingernails have two sets of ridges. You have vertical ones which go along the length of the nail and those ridges are just caused by variation in the cells that line your nail bed. You can also get horizontal ridges and I think these are mainly thought to be caused by trauma to the nail bed. You can also get illness events which cause all sorts of interesting things to happen to your nails. You get white ridges and the little white marks as well but I think that's generally attributed to trauma. Chris - I was thinking about why those marks might be white. I was thinking it might be similar to the reason that when you smash glass, or grind up glass and make sand again, or you make water freeze and make little ice crystals like snow - it's white, whereas the thing it started out as was translucent. If you look at the nail, it's a crystal structure of keratin, the protein, and if you traumatise it you probably get different fragments of bits of protein and water in there and other debris, all of which probably bend light all over the place and does the same thing as snow - so it reflects more light back, so you get a white patch. Diana - Yes, a lot of people seem to talk about zinc deficiencies or various other vitamin or mineral deficiencies that cause this but it seems to be something more trauma related.