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Heads up—this is a pretty gross question.

The dragons in this world are not reptilian, but rather resemble huge borzois. I've not sure how large I want them to be yet, but they are capable of mining/retrieving raw crystals themselves, so I imagine they must have paws like steam shovels.

So what I'm asking is:

  1. What would these things excrete, if anything?
  2. What kind of mechanism would they need to digest minerals or convert them into energy?
  3. How much excreta would they generate, if any?

Many human villages in this world are built near dragon habitations, so I feel their bodily functions would probably affect people's lives just a little bit. That's why I'm asking.

Answers/suggestions don't have to be scientifically rigorous. I will accept ideas that boil down to "magic." Thanks!

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    $\begingroup$ Falcor! $\endgroup$ – Mazura Jul 15 '15 at 22:32
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    $\begingroup$ maybe due to the unique diet your dragon need to gobble up tons of diamonds and emeralds to break down food whereby your common rocks and pebbles would instantly dissolved by its gastic acid. $\endgroup$ – user6760 Jul 16 '15 at 2:56
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    $\begingroup$ oh I almost forgot the magic part is your dragon's stomach lining. $\endgroup$ – user6760 Jul 16 '15 at 3:13
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    $\begingroup$ Just wondering, why are you calling them "Dragons" if they aren't similar to what is usually gathered under the label of dragons? Wouldn't that confuse your readers? Why not call them Earth Hounds, Mole Hounds, or give them a new label instead? $\endgroup$ – Murch Jul 16 '15 at 13:39
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    $\begingroup$ It is worth pointing out - because all of the answers are wrong on this point - waste is, by weight, 75% water. Even if eating rocks and gems, the dragon will not be pooping dust, as everyone here suggests. The rest of the poop that isn't water is mainly dead stomach bacteria, and dead human cells. It is NOT mainly undigested stuff we ate. Now since we are talking magic, it can poop out anything. You could have their intestines turn lead into gold if you wanted. But if you are trying to minimize the magic, the poop will end up being poop. $\endgroup$ – Shane Jul 16 '15 at 17:33
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Gemstones are crystalline minerals. A diamond is made of 'plain ordinary carbon'. The atoms are just arranged in a particular way to create a crystalline lattice. Other crystals are different elements, but there's nothing particularly special there. Looking at the chemicals involved:

Emerald: $Be_{3}Al_{2}(SiO_{3})_{6}$

Ruby: $Al_{2}O_{3}Cr$

Diamond: $C$

Quartz: $SiO_{2}$

Given enough time (millennia) diamonds can naturally turn back into graphite.

This is largely true of all crystalline materials - they're not actually particularly special chemically speaking. So I'd suggest the 'excreta' would be 'mineral dust'. Think 'coalmine slag' - where the 'useful' coal is gone, and all that's left is black dust.

For digesting and using the minerals - that's actually quite tough. Not many creatures eat inorganic compounds. Some do because it assists digestion (food grinding) or minerals that aren't readily available (iron supplements).

Neither of these particularly would require crystals though - probably more the opposite - the things that formed them into crystals make them harder to digest. You'd need some serious pressure or heat to cause them to decompose enough to 'break down'.

But maybe that's why they need the minerals? If they're dragons, that implies some pretty strange biological processes in the first place, so maybe 'normal' minerals get used up too early?

One possiblity though is that used by e.g. Anne McCaffery in Dragonriders - the firestone is eaten not for nutrition, but because it contains the essential ingredients for breathing fire.

A somewhat tenuous thought though - carbohydrates are basically carbon, hydrogen and oxygen. What if your dragon 'lived' by a synthesis mechanism, where it breathed in some of the minerals it needed (hydrogen, maybe oxygen?) but couldn't get at a ready source of airborne carbon? Carbon dioxide is readily available in the air, but free carbon isn't.

So perhaps - it 'eats' minerals to combine and 'make food' a bit like plants do. It'd be the carbon and oxygen that was 'of interest' there primarily. These might even be naturally occurring as e.g. coal, but diamonds might be denser or 'taste better' or metabolise slower.

And as for excreta - by volume, whatever isn't 'used' has to be displaced. Maybe you'd have less for 'pure' carbon (diamonds) but might have a silica residue if the dragon had been eating quartz recently.

(This is somewhat speculative of course - this isn't 'real' chemistry by any means. Some degree of 'handwavium' will be required therefore to stretch this into a plausible shape).

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Complely non-science based answer (due to a lack of being able to come up with a science-based way anything could draw nutrients of gems):

Make your dragons a silicon-based life-form. Biological creatures need organic matter, a mineral based creature would eat minerals (note: this is not how actual, exobiological silicon-based life would work!)

They extract crystalline molecules from gems to create their "flesh" and "bones"; they consume coal, oil, and wood to fuel the fires which are an important part of their digestive furnace, in addition to allowing them to breath flame. Probably will also use rocks as "grist", if the digestive system both grinds and heats.

They could excrete a crumbly mixture of sand and ash, all that's left after they've absorbed everything they need from the minerals.

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    $\begingroup$ Better than bones - how about have an exoskeleton? Maybe the mineral deposits 'exude' into the scales, to 'harden' them, and the dragon that's been most mineral-crunchiest is also the best armoured.... $\endgroup$ – Sobrique Jul 15 '15 at 16:20
  • $\begingroup$ @Sobrique -- I like that! $\endgroup$ – LindaJeanne Jul 15 '15 at 16:21
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Have you considered something like existing dirt-eating worms? The tailings move out as it goes, the animal essentially filtering the dirt in-place. Worm castings are valuable to gardeners, whether locally produced or introduced as a fertilizer.

Bird and bat guano is quarried and sold, as fertilizer.

I'm thinking thag (1) the excrement could be a valuable resource, and (2) it could be rather different from what we normally think of.

It would be a more interesting story if it wasn't a mundane solid blob of waste remaining from what was eaten. What if it was all magicly transformed, but not all kept for use? The excrement could be gems made by the digestive system from base rock, or it could be expanding foam that makes a good building material, or something like volcanic pumice only with residual magical properies as well.

Let me run with that: after consuming several tons of rock that is magicly stored in the digestive system without taking up much space or adding weight to the animal, he moves to the edge of his territory and excretes a blob that is initially the size you expect from an animal that size, but is the color of magic and potently magical: touching it causes awful death from uncontrolled magic applied to the body in random ways, and the fumes can cause magical effects too.

The material quickly developes a stable coporial form that's like tar or melted glass, but is only mildly hot, and no longer deadly with uncontrolled magic. Then, gas produced inside the mass makes it expand into a huge quantity of mukti-colored foam, and after a day or so the material hardens and is like foamed rock or glass, exactly what depending on what was eaten (both base minerals and magical gems). It is like pumice or slag, but also has some residual magic, and a skilled practitioner can guide it to various useful magical properties as it cures. This can be enchanted building material, with other uses developed over time.

The dragons use it to make lairs and nests like termites, and can control the magic cureing process to that end.

The dragon guano craftsmanship is very dangerous, as working with it early in its wild state gives more potential, and accidents happen. Occasionally random magic has some strange effect nearby. So, the craftsmen also arrange various items around the work arena: staues of dieities, amulets, totems, and blank paper. Once or twice a year an anomolous discharge enchants one of thes objects which acts as a lightning rod: the wild formless magic is guided by the nature of the magical item it is to produce, rather than doing something completely random. And, the charmed item, if identified, can be a valuable by-product.

Just as ancient dye makers (strong bases), felt makers (mercury), and even washer-women (lye vapors) had characteristic occupational hazards, so do the scatmorphers.

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Magic physics imbues materials with the same energy that created them

From a certain point of view, it makes sense. Plants contain energy because they capture the power of the sun and turn it into carbohydrates; animals gain energy by eating those plants.

Using the same logic, many gems are created from heat and pressure deep beneath the surface of the earth and therefore "contain" the power and energy of the plutonic depths, which can be extracted by certain magical processes. The most valuable gems (diamonds, rubies, and sapphires) are typically formed from extreme heat and pressure.

Some gemstones, such as quartz crystals, do not require extreme heat to grow, but they do require extensive amounts of time. These crystals can act as reserves for the latent magical energies that were around over the time it took for them to form.

Almost all valuable crystals are formed from heat, pressure, or time, which is what differentiates them from mundane rock.

To be fair, some kinds of non-gemstone rock (such as marble) also forms from extreme heat and pressure. Perhaps this form of magic tends to "leach out" of large layers of stone, only small, concentrated crystals can contain the energy properly. Maybe a high surface area to volume ratio is good for magic storage, for whatever reason.

Mages in this world may use gemstones to power their magic, but dragons utilize the same energies in a more direct manner. When a dragon breathes fire, this fire is powered by the same energy that was used to create the gems they consumed. This can even be used as a sort of dichotomy - most living beings get their energy from the "upper fire", the sun, while dragons and mages draw their energy from the "lower fire" within the earth.

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Requirement: Your creature digs for Geodes to eat. Geodes contain iron, magnesium, sulfur, amongst other elements that your creature needs to digest to survive.

1) What would these things excrete, if anything?

Your creature would excrete dusty dung filled with crystals. Dusty because of the extra stone dust that is not processable. Geodes often contain crystals such as Amethyst, Quartz, or Pyrite. These excretions could be farmed by your villages to obtain these valuable stones, and used for trade/crafts.

2) What kind of mechanism would they need to digest minerals or convert them into energy?

This part is magic. As far as I know, there is no way to actually turn rock into nutrients. Give your creature a magical stomach to draw out the nutrients required from the Geodes. This process just so happens to leave any fully formed crystalline structures intact, so that your creature can poop out gems.

3) How much excreta would they generate, if any?

This really depends on your creature. The average Geode doesn't contain nearly enough nutrients to sustain your creature, unless your creature hibernates for most of the year - if this is the case, then there would be poop during certain seasons (a lot of it while it stores up nutrients, since there are usually a lot more crystals in a Geode than there are nutrients). However, if your creature eats year round, then your creature may need to be smaller so that it can sustain itself on Geodes in the area. In this case, there would still be poop, just smaller batches of it found more frequently.

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Diamonds, rubies and possibly emeralds will burn rather energetically. That means there is rather a lot of energy in them. The dragons would need some kind enzymatic process to unlock that energy.

Probably, they would be ground by rubbing them against each other in some kind of gizzard. The walls of the gizzard might be coated in stones.

Once pulverized, the gem dust would be moved into a stomach that would have a mild electrolytic solution. The movement of ions would assist the enzymes in attacking the energy rich bonds of the gems. Some of the energy could be recovered from the electrolyte. The carbon could be utilized in a modified Kregs cycle. The various metal ion could be used in a modified mitochondria.

The carbon would be exhaled as CO2, the SiO3 and perhaps the Al and Be could be integrated into bones. The excretion would likely be something that looked like sand although the metals might possible be excreted as pure metal modules.

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I'm rather sure the actual digestion part is going to have to be summed up as 'magic'. Since that's the case, why does a magical creature necessarily have to excrete anything?

So your creatures eat rocks - especially shiny rocks - to generate energy. We're already in the realm of the fantastic at this point. Give them a place they can excrete from in the case they eat something that they can't magically digest - a magic sword, a unobtainium time capsule, whatever - but they're far too mighty, majestic, and above all perfect to lower themselves to leaving piles of stinking dung everywhere. Their metabolism consumes rocks, gems, metal ores, small animals, large animals, animals that like to think they're not animals, the weapons and equipment of the aforementioned non-animal animals, and the hopes and futures of many a would-be dragonslayer; and it converts it ALL into energy.

(Note that most things we'd consider dragons shouldn't be able to fly under their own power without magical help anyway, I'm assuming that's what the gems are for. A magic metabolism isn't a stretch. 2e AD&D dragons could already eat rocks if they couldn't find anything else. One of my players played a "vegetarian" gold dragon once that only ate rocks unless they actually needed help clearing overgrowth or jungle or something.)

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Do you want your dragons to eat the crystals for energy? An alternative would be that they eat them as a way to accumulate building material they can not synthesize otherwise. Maybe they need the crystals to lay eggs ? Or they build a hide-out like a bee-hive ? It would surely be an interesting place for adventurers to seek out such "crystal-hives" or nests with crystal-eggs.

One way or another, crystals are generally rare, so any digger would have a lot of excrements which are just 'filtered/homogenized' dirt. Maybe these dragons "drill" tunnels which (from their backside) are then filled by sand? Such sand-tunnels could be potentially harmful if drilled vertically, but creative humans might find some usage...

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