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It ought to be some combination of C, O, and H, I presume. But really why should I care if eggshell gets in my eggs while I'm cooking?

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Could you specify which animal's egg shell ? –  biogirl Sep 21 '13 at 19:48
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Sure eggs from hens –  Rustyn Yazdanpour Sep 21 '13 at 19:49
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Just google eggshell :) By the way, it is mostly calcium carbonate stabilized by a protein matrix. I have heard that excess calcium carbonate may cause some side effects but I don't have any good source. –  biogirl Sep 21 '13 at 19:52
    
@biogirl I don't want to google things; I would like to ask questions here to build a larger index of Q&A. Sure, I could always google things, but, asking questions here in the fashion that I have, is a catalyst for participation. If everyone feels that I should seek to answer my own questions through google, (which I am perfectly capable of doing), why have biology stack exchange? I would rather get the unique, human, and academic perspective from biologists and biology enthusiasts alike. –  Rustyn Yazdanpour Sep 22 '13 at 19:35
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@RustynYazdanpour you should do a google search. From the help pages biology.stackexchange.com/questions/how-to-ask: "Search, and research. Have you thoroughly searched for an answer before asking your question? Sharing your research helps everyone. Tell us what you found (on this site or elsewhere) and why it didn’t meet your needs. This demonstrates that you’ve taken the time to try to help yourself, it saves us from reiterating obvious answers, and above all, it helps you get a more specific and relevant answer!" –  GriffinEvo Sep 23 '13 at 9:43

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up vote 6 down vote accepted

It's the same stuff that makes up most sea shells, calcium carbonate, $CaCO_3$.

Incidentally, this explains why egg shells dissolve in vinegar (acetic acid which, since it is an acid, provides the Hydrogen ions in the reaction below):

$CaCO_3+ 2H^+ -> Ca^{2+} + H_2O +CO_2$

This simple reaction (which is what would happen in your stomach as well) produces water, calcium ions and carbon dioxide, none of which are particularly harmful.

The specific reaction with acetic acid is

$CaCO_3 + 2CH_3COOH → Ca(CH_3COO)_2 + H_2O + CO_2$

which also produces calcium acetate.

Anyway, there is nothing particularly toxic about $CaCO_3$, it's basically chalk. I wouldn't recommend eating a lot of it but you shouldn't freak out if some falls into your omelet either.

However, don't forget that an egg has exited a chicken's anus (well, cloaca) so their shells are likely to have come into contact with feces and they are porous so they are not necessarily the cleanest thing around. This is just not because of any chemical toxicity.

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I thought that the calcium carbonate dissolves in olive oil, and calcium acetate forms. Should I have anything to worry about as far as calcium acetate goes? –  Rustyn Yazdanpour Sep 22 '13 at 19:16
    
@RustynYazdanpour calcium acetate is harmless. As you can see in that link even at very high concentrations (like those you will get if you buy it from a chemical company) it is an 'irritant'. In the concentrations you might find in your kitchen you can safely ignore it. –  terdon Sep 22 '13 at 19:27
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@Rustyn Yazdanpour: I doubt you'll get acetate from olive oil... it would rather come from the reaction terdon wrote (where he omitted acetate ions). In any case no worry about that either, although there are definitely much better uses for eggshell (composting for instance). –  nico Sep 22 '13 at 19:28
    
@terdon Ok, this is a full comprehensive answer. I really hope that the kinds of questions I'm asking are appropriate for this forum. Otherwise, if the community would prefer, I can ask some of these questions on chemistry stack exchange. –  Rustyn Yazdanpour Sep 22 '13 at 19:29
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If you don't get any more chemical than this, you're fine. If you are asking about specific reactions chemistry might be better. If it is about their effect in a living system, they belong here. And no worries, you're very welcome :) –  terdon Sep 22 '13 at 19:31

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