I have the strangest cravings sometimes. I love chalk. That powdery substance smells so good. The smell of a musky basement makes my mouth water. Fine powdery dirt on a windy day makes me lick my lips. I used to think this was a completely normal human craving until some friends informed me that this is quite odd and indeed deviates from the human definition of normal. The craving for non nutritious substances is called Pica.
Since then I have learned that it is not completely known what causes pica, but some cases may be related to a mineral deficiency. I would believe this because after going on long runs or hikes, I generally crave chalk more.
However, it seems there are several different categories of pica that go into the realms of odd and strange. There are even names for the various forms of pica, which comprise such categories as the consumption of paper (xylophagia), glass (hyalophagia), or even feces (coprophagy). Luckily my cravings only consist of mineral-like substances.
Recently I had the chance to go to the Glore Psychiatric Museum in Missouri. There is a display there (and a picture of it on Wikipedia) that shows the contents that were found in the stomach of a psychiatric patient after an emergency surgery. Below is an image of the display, along with some of the descriptions.
Just to reiterate, this person had 1,446 metal objects inside them. While your stomach acid is strong, it does not act quickly enough to dissolve entire pieces of metal.