These can mean a few things, says Jessica Krant, M.D., a board-certified dermatologist in New York City and founder of Art of Dermatology. First, it could be just a natural, genetic pigmentary change, like a freckle. But if there's a dark streak along the nail from cuticle to tip and there's only one—or it's changing fast—it may mean something more serious: melanoma of the nail, says Krant. This is a form of skin cancer and potentially deadly. Some nail fungus infections can also be dark gray or green.
And by that, we mean blue-ish nail beds, which signify that your fingertips aren't receiving enough oxygenated blood, says Krant. This can be a sign that your circulation is bad in your hands and feet or that your lungs aren’t properly oxygenatating the blood in your whole body—either due to lung disease or heart disease.
These can mean several bad things, but the most serious thing to worry about is liver disease, says Krant. So if you notice this, make sure to bring it up with your doctor.
Thin, peeling, or spoon-shaped nails (ones that are curved in a concave instead of convex way) are associated with iron-deficiency anemia, says Krant.
If you have nails that are hard but break easily, this can be a sign of dryness or possibly hypothyroidism—especially if you also have thinning or unusually dry hair, says Krant.
These usually develop normally with age. "It's a sign that the nail matrix (the root of the nail under the cuticle) is drying out," says Krant. "Sometimes it's possible to make it better by keeping extra-heavy moisturizing emollients and ointments on the cuticles and nails."