Keratin

A natural fibrous protein that is the primary building block for epidermal tissue. Keratin contains all of the common amino acids, but has higher amounts of cystine than other fibrous structural proteins. Soft keratins are found in the external layers of skin, wool, hair, feathers, while hard keratins are the major components of tortoise shells, nails, hoofs, scales, bills, and claws. In addition to usage in their natural form, since the mid-1600s keratin has been molded into buttons, combs, and buckles.