Hard skin. Coloured scanning electron micrograph (SEM) of hard skin on a toe. Hard skin occurs due to hyperkeratosis, an overproduction of keratin proteins in the skin that leads to thickening. Hyperkeratosis is a result of pressure on skin over bony prominences. It is most common on the balls of the feet, the heels and the knuckles of the toes. As the skin continues to thicken, corns and calluses form. Magnification: x1700 when printed at 10 centimetres wide.