Normal skin is 80% collagen and 4% elastin fiber network. Elastin is the protein in the skin that gives it flexibility and allows it to stretch and recoil back to its original position. Stretch marks, otherwise known as striae distensae, are a form of discolored scar found in the skin, in which the elastin fibers have been irreversibly damaged. Since our skin cannot repair elastin fibers, stretch marks do not heal.