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As far as I know, most of the "hard things" in living creatures composed of calcium (for example, bones plated with calcium over the keratin structure, and eggshells and seashells with $\ce{CaCO3}$). The calcium ion has an oxidation state of +2, and so does iron.

Why don't we find in nature creatures with bones of iron or eggs with iron shells, given that living creatures do use ionic iron for other purposes like in the heme group of hemoglobin?

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    $\begingroup$ iron would get oxidized, whereas calcium wont get oxidized as calcium has a maximum oxidation state of 2+. Iron could get oxidized to its 3+ state, ie, it would rust reducing the strength of the bones. $\endgroup$ – Prakhar Nov 20 '16 at 14:14
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    $\begingroup$ Structures made form calcium carbonate can be really tough (bones are stronger than concrete) and are much less dense than iron. Plus calcium is more abundant than iron in food. $\endgroup$ – vapid Nov 20 '16 at 14:30
  • $\begingroup$ @vapid "Plus calcium is more redundant than iron in food" is a circular argument: The reason it is not abundant in food is the very same reason why it is not used in nature. $\endgroup$ – logical x 2 Nov 20 '16 at 16:13
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    $\begingroup$ Iron eggs? I guess little birdy would have a hard time trying to break out of an iron egg, just saying ... $\endgroup$ – logical x 2 Nov 20 '16 at 16:16
  • $\begingroup$ biology.stackexchange.com/questions/9419/… $\endgroup$ – Mithoron Nov 20 '16 at 23:21

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