If dirt, as William James put it, is matter out of place, then the dirtiest dirt of all is the kind you put where you’re absolutely not supposed to: in your mouth. We teach children not to eat dirt even before they can talk; conversely, telling someone to eat dirt is a powerful expression of contempt, a way of demoting them from human to animal. Yet as Sera L. Young explains in her quirkily informative book Craving Earth: Understanding Pica, eating dirt—in particular, certain kinds of dry, crumbly clay, as well as other non-food substances like uncooked starch, chalk, and ice—is a very widespread human practice, and always has been. Pica, as this behavior is known—the name comes from the Latin word for “magpie”—is especially common among pregnant women.
This has been recognized since ancient times: Hippocrates, the 5th-century BC Greek physician, noted that pregnant women often had cravings for earth or charcoal, and a classic Indian poem describes a pregnant queen who “set her heart upon clay in preference to all other objects of taste.” Today, Young reports, Americans with pica buy boxes of chalk at Walmart, or bags of ice (the cubes at the Sonic fast-food chain seem to be especially popular), or even order prime Georgia dirt over the Internet.
http://bnreview.barnesandnoble.com/t5/Reviews-Essays/Craving-Earth/ba-p/4601
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