Dirt substitute? It's been suggested that we're genetically wired to crave gritty or crunchy substances when we have iron deficiency because dirt and clay contain iron. (ice does not, but may satisfy this instinctive craving anyway.) however, this folk wisdom is controversial, and others have suggested that chewing ice simply relieves the tongue discomfort that can occur with iron deficiency.
Pagophagia. The exact relationship between chewing on ice and iron deficiency has not been exactly worked out. Many physicians will order iron studies on patients that report chewing on ice just to make sure there are no deficiencies. Further studies will be needed.