What if rocks tasted good and had nutritional value? Would you eat them? Would you grind them into powder and pour them on your cereal?
Some rocks of course, are edible, and nutritional, but the taste leaves something to be desired (that something is probably real food). However, the rocks that you can eat do not by any means taste good, and if they are eaten whole, they'll be the last thing you ever eat (teeth are nice to have, but rocks and teeth don't get along as well as they should).
Whether you think eating rocks is a bad idea, good idea, or something you are currently doing while reading this, you have at one point or another eaten rocks in your life. The denials have started. But really, you have. Every time you have a beach picnic. Every time you eat near dust, and whenever you have a vitamin, or vitamin enhanced orange juice or cereal. The first two seem obvious, but the second two not so much. The fact is, zinc, calcium, copper, iron, etc. all come from rocks, and get into our food. With good and bad results. The Romans liked rocks with lead in them, and even made pipes out of the stuff.
So be careful when eating rocks. You can't avoid eating them, so just be careful. And eating them is sometimes just as rewarding as throwing them.