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So, it is relatively common to have various creatures grow crystals. This generally happens outside of the body (at least partially), but internal crystals aren't unknown either (see for example, every non-earth-native animal from Brandon Sanderson's Stormlight Archive, which use a gemstone to collect and store magical energy).

Now, I guess you could say that kidney stones and the like might qualify as crystals, but those are more of a by-product, that is in fact harmful.

My question is, why would an animal (or plant) choose to grow a crystalline structure? As far as I know, organs are generally organic, with veins going all over the place on the inside. A crystal wouldn't allow for that.

Why would a body invest resources in growing a crystal?

Edit: Note that while bones do include crystals, they aren't exactly visible without a microscope, from what I can tell. What I'm looking for would be a crystal that is formed biologically, and is big enough to see and recognise as a crystal, without any microscopes and stuff.

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    $\begingroup$ Gemhearts in the Stormlight Archive are magic based, as you pointed out. Asking for a biological reason for a magic construct is kind of pointless - the answer's just magic. $\endgroup$
    – Halfthawed
    Commented Mar 26, 2020 at 16:14
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    $\begingroup$ Tooth enamel is mostly crystalline hydroxyapatite; all vertebrates make it. The skeleton of echinoderms, such as starfish, is made of crystalline calcite. Are more examples needed? (Note that you may be using the word "crystalline structure" with a meaning different than the one used in physics and mineralogy. You may think of some sort of faceted transparent object; such object may be or may not be made of a crystalline substance. For example, crystal glass is not actually crystalline.) $\endgroup$
    – AlexP
    Commented Mar 26, 2020 at 16:16
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    $\begingroup$ @Halfthawed well, the reason I brought them up was a ss an example for interior crystals. My question is why would they do this if there wasn't any magic around. $\endgroup$ Commented Mar 27, 2020 at 7:54
  • $\begingroup$ @AlexP Huh. for some reason, I thought that bone structure was more... organic than that. What's the word... heterogenic? Mixed up. I mean, veins do go in there, and feed the osteothingumys, so I thought that whatever crystals there would be would just by inside the cells. $\endgroup$ Commented Mar 27, 2020 at 7:56
  • $\begingroup$ @MarkGardner: The ossicles of echinoderms, and the tooth enamel of vertebrates, are not made of bone. (And the mineral part of the bones of vertebrate is in the extracellular matrix, not inside the cells. Vertebrate bones consist of living cells embedded in a non-living extracellular matrix. The matrix is made up of 1/3 collagen for elasticity and flexibility, and 2/3 minerals, mostly hydroxyapatite, for strength.) $\endgroup$
    – AlexP
    Commented Mar 27, 2020 at 9:44

2 Answers 2

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Here's the thing: we living things already grow things with a crystalline structure.

And I'm not just talking mollusks here. Anything with a skeleton can and does.

So bones. And here's some linkage. And a little more linkage on the actual structure.

For the most part, the human body does not contain minerals. Scientifically speaking, a mineral is a naturally-occurring inorganic crystalline solid with a single chemical formula. Rocks are aggregates of minerals and organic materials. Except for in bones and teeth, the atoms and molecules making up a healthy body are not crystalline and are not solid[...] Bone mineral is indeed an inorganic, crystalline, solid with a single chemical formula and therefore qualifies as a genuine mineral. The mineral in your bones is called hydroxyapatite and has the chemical formula Ca5(PO4)3(OH). Our bodies build bone mineral on the spot, so we don't have to swallow hydroxyapatite crystals. But we do have to eat food with enough of the right kinds of atoms to build bone mineral. Looking at the chemical formula, we see that our bodies can't build bone mineral unless we supply it with enough calcium, phosphorus, oxygen, and hydrogen.

Your question, I believe was: Why would a body invest resources in growing a crystal?

And the answer is this: any animal that requires a structure that is solid, yet can change will likely have something crystalline in nature. Bones, teeth, shells all of them are inorganic and qualify as crystalline. A body invests resource in growing crystals where a structure is needed to support that body and have it work properly (skeleton) and also as grown inorganic tool (teeth, which are both organic and inorganic).

My suggestion is to look at how all of those form and use them as a jumping-off point for whatever you are trying to do. The reason why crystalline structure elsewhere (like the kidneys) is bad is because it's a structure that doesn't belong, material forming in a way that does NOT support the FUNCTION of the organ and organism.

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  • $\begingroup$ Ah, thanks for pointing that out. But don't these things all contain veins, with cells on the inside to strengthen or weaken the structure? I wouldn't say that the whole bone or tooth or shell would qualify as a single crystal. Sorry for moving the goalposts on you, I hope you understand. $\endgroup$ Commented Mar 27, 2020 at 8:05
  • $\begingroup$ For real you should be looking at how bones form. It's the closest we have. It would be unrealistic for any crystaline structure to grow without being fed by the organic system in some way. Just because they don't look like what you want on a marco level doesn't mean they won't be informative on a micro-level. (Also, what you might think is a 'single crystal' is generally a lot of crystals made to form what looks like, to you, a single crystal.) $\endgroup$ Commented Mar 30, 2020 at 4:01
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Eliminate waste products

Gout is caused by crystals forming inside your body. An imbalance causes the substance that makes up the crystal to build to a level it's not supposed to get up to, which causes it to spontaneously grow crystals (much the same way that rock candy crystals spontaneously form on the string, if there's too much sugar in the water).

Since the crystals aren't supposed to be there, this is excruciatingly painful. However, let's turn that around. At concentrations which are not fatal to the creature, these crystals form. Suppose a creature's waste disposal system were designed to operate this way - the creature has a way to get the crystals to grow where it wants them. Then the crystals will naturally grow just from being hooked up to the circulatory system (or the equivalent of the kidneys, or what have you).

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  • $\begingroup$ This was where my thoughts went. Crystals on the skin could even have unintended benefits, like poisoning predators or being spikey and protective. Or the crystals could be an abnormality of excretion like a plugged duct or a pearl. $\endgroup$
    – DWKraus
    Commented Apr 2, 2020 at 19:29

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