The calluses are horny layers that form in certain parts of the skin because of the call hyperkeratosis, which causes the upper layers of the skin to thicken by an increased presence of keratin. By losing moisture, nerve endings and tissue atrophy becomes harder and insensitive to pain. If this skin is exposed to constant pressure and friction will form a strong yellowish surface will form a callus.
The calluses vary in shape and size. They are usually painless, but in some cases become so thick that the skin loses flexibility and break causing discomfort. People with diabetes or poor circulation may develop serious infections under calluses.
The calluses on the soles or heels are caused by wearing shoes that press your feet too. People who go barefoot all the time and in all types of terrain, often develop calluses up to 1 cm. thick.