Fingernails provide subtle clues about our overall health — though they're often either covered in nail polish or overlooked altogether.
They're made out of keratin, which is the same material in found in skin and hair.
According to Allure, weak and brittle nails are often a sign of deficiencies in iron, or in vitamins B or D.
Horizontal ridges can come from physical trauma, but can also indicate malnutrition, psoriasis, or a thyroid problem. See a doctor.
A brown, vertical stripe is called melanonychia. While usually benign, it can sometimes indicate melanoma. See a dermatologist immediately.
Finally, if one or more of your nails is red, swollen, and infected, go to the emergency room right away.