Are cravings in fact caused by your body's need for some obscure nutrient? Apparently not:
Food craving, defined as an intense desire to eat a specific foodstuff, is a common occurrence across all cultures and societies. These yearnings, and those associated with nonfoodstuffs such as pagophagia (the practice of consuming ice) and geophagia (literally earth-eating), are not linked to any obvious nutrient insufficiency. In some individuals food cravings and dietary restriction may be related; however, these observations are inconsistent with the majority of published studies.
The article goes on, in classic Scientific American style*, to try and explain what might cause cravings. The usual suspects-- hormones, psychoses, cultural traditions, etc., are discussed, but it would seem no single source has been found to-date.
* A sort of "nerdy-scientist-wanting-to-be-popular-but-not-talk-down-to-the-plebes" tone. Sometimes it works, sometimes it doesn't. Since SA nearly always publishes stuff from the primary source, the "I performed experiment A to determine..." and "our studies in clinical trials have shown..." can be a little disconcerting as well.