Cracks on the heels and the side of the feet are often a problem in warm countries were bare feet or sandals are the norm.
However for cracks to become deep and fissured there is often an underlying deficiency of Zinc in the body.
There may also be deficiency in B vitamins and/or Vitamin A (check for dryness of the lower lip).
Try taking a vitamin B complex tablet twice a daily (breakfast and lunch time is best) plus extra Zinc, or better still one tablespoon of Colloidal minerals daily.
The use of pumice stone certainly helps prevent the formation of deep cracks on the heels. Use a pumice stone in the shower, and then rub on an oil rather than a moisturiser.
Castor oil also works quite well, although it is very sticky, as it is very good for softening hard skin.
Other creams/oils that are Vitamin E based and/or have a antibacterial/antifungal effect can also be helpful if used regularly.
However do make sure that the feet are kept dry and well aired especially if having to wear shoes/boots for most of the day.
Vitamin and Mineral supplements need to be taken long-term for good results.