Heel calluses, also called "cracked heels," have many potential causes. Dry skin (xerosis) is common and can get worse with wearing open-back shoes, increased weight, or increased friction from the back of shoes. Dry cracking skin can also be a subtle sign of more significant problems, such as diabetes or loss of nerve function (autonomic neuropathy). This can also be a presentation of Athlete's Foot (Tinea Pedis).
Heels should be kept well moisturized with a cream to help reduce the cracking. If an open sore is noted, make an appointment with a foot and ankle surgeon for evaluation and treatment.