Craving and eating nonfood items is a condition called pica and those with the disorder are known to compulsively eat paint, feces, cigarette ashes, paper and a variety of other non-nutritional items. When the cravings are for soil or otherwise earthy materials (including coal, chalk and clay), it's called geophagia.
Pica and geophagia may be caused by nutritional deficiencies of iron or zinc, often because of dieting, food deprivation or malnutrition, but may also be symptoms of developmental disabilities such as autism or mental retardation or mental illnesses such as obsessive-compulsive disorder. Some women experience pica during pregnancy, and some cultures don't find such practices out of the ordinary.
Pica may be treated with a combination of behavior therapy, aversion therapy and positive reinforcement. Environmental education, family education and medical treatments are helpful in addressing any nutrient deficiencies, anemia or lead exposure.