Healthy natural nails are primarily made of a protein called:

Healthy natural nails are made primarily of keratin, the same tough, fibrous protein that makes up hair and the outer layer of skin. The hardened keratin gives the nail plate its strength and protective quality. (Collagen and elastin are proteins of the dermis; melanin is the pigment that colors skin and hair.) Knowing that both nails and hair are composed largely of keratin is a basic but frequently tested fact connecting nail and hair science on the cosmetology exam.