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In my Fantasy world I thought it would be interesting to have a creature whose eyes would be precious gemstones or crystals. I want to understand how that could be possible, and if any ramifications could come of this. The world is not highly magical, so a non-magical solution is desired.

For instance, I know that there are clams which can produce pearls by slowly accumulating minerals and polishing them into a smooth pearl. I am thinking it could work in a way similar to that.

The creature would be born with tiny eyes made of only a very little bit of some mineral. This creature would live in an area rich in this mineral, and it would be accumulating this mineral in its diet over a long time these minerals would deposit in the eyes to make them grow.

Now the problem is how would the creature be able to see with these eyes?

I thought maybe it could work like a lens or something similar, with some organ behind the gems that catches the light and the gems or crystals refract it.

I am curious if this is something that is possible at all, and if so - how would it impact this creatures sight?

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    $\begingroup$ Most precious gemstones I know of are formed by pretty intense pressure and heat. You might get by with your creatures growing crystals internally though. $\endgroup$
    – user10945
    Commented Jan 4, 2017 at 9:48
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    $\begingroup$ To add wit PETE it is also often that only a polished gemstones look good. Crystals are not gemstones. With a fantasy setting your creature could utilize them similarly to camera, the film being the organ. $\endgroup$
    – user3644640
    Commented Jan 4, 2017 at 10:52
  • $\begingroup$ Good point, I had not considered that Crystals could be a choice too. Ultimately its the "precious X" that I wanted, gem or crystal makes no real different to me. I will edit. :) $\endgroup$
    – Inbar Rose
    Commented Jan 4, 2017 at 11:00
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    $\begingroup$ Gemstones were used instead of glass to make lenses, back when optical glass was not available. Emperor Nero is said to have used an emerald lens to correct his short-sightedness and enable him to watch gladiatorial games. What humans can do nature can do too, especially in a fantasy setting. $\endgroup$
    – AlexP
    Commented Jan 4, 2017 at 13:15
  • $\begingroup$ I need to disagree with precious gemstones being formed from intense pressure and heat. From nature.berkely: "Geologic conditions of formation [of topaz]: Topaz crystallizes from fluorine-bearing vapor in last stages of solidification of igneous rocks. Thus, cavities in lavas and granitic rocks in pegmatites, and in alluvial deposits. Secondary concentrations of topaz occur in stream beds and other alluvial deposits." If in a fluorine rich environment, this creature could form eyes from a 'seed' gem in the base of the eyes. Maybe a chemical process involving acid to create the heat? $\endgroup$
    – BaseHobo
    Commented Jan 4, 2017 at 18:29

5 Answers 5

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It could work very similarly to, but potentially even better than, modern land-based vertebrate eyes. Land-based vertebrate eyes have four to eight main parts.

enter image description here

First, there is a thin tissue layer with liquid behind it (the cornea). This does most (about %70) of the focusing of the light (cornea). Second, there is another flexible tissue layer (the lens) which does some focus but more importantly can change the focus of the eye by having its shape changed by muscles. Fourth, between these two is a muscular ring (the iris) that can quickly change how much light comes in. Fifth, there is a layer of cells that (among other things) convert the light into cellular responses (the retina). Sixth, many animals (but not mammals) have small colored fluid-filled bubbles in front of some cells that improve color perception. Seventh, some animals have a reflective layer behind that improves the amount of light collected in dark at the expense of lowering the clarity of the image (tapetum lucidum). Finally, many animals (but not primates) have a transparent, retractable covering for the eye called the nictitating membrane.

Crystal could replace the cornea, tapetum lucidum (if it is active a lot at night), and nictitating membrane without any changes at all. It couldn't replace the current flexible vertebrate lens, but other animals have a rigid lens that is focused by being moved forward and back. The crystal could work for this sort of lens as well. If the crystal has other minerals added to it to give it color it could also replace the fluid-filled bubbles in front of cells.

It could also improve over the existing vertebrate eye. Our eyes have a problem called "spherical aberration", where the focusing of the eye is not exactly the same at all points, and "chromatic aberration", where the focusing is not exactly the same for all colors. Spherical aberration is why we squint to see better.

In our precision optical instruments like telescopes and microscopes, these problems are addressed by making lenses with multiple layers of different materials with different optical properties. This allows us to much correct for spherical and chromatic aberration much better than living eyes. Crystalline eyes could use a similar mechanism, where layers of tissue, fluid, and crystal with different properties could be used in the cornea and lens to provide a much clearer image than any existing eye.

This has the added benefit would require a much thicker cornea than land vertebrates, making both the cornea and lens suitable as gemstones. The multiple layers and optical properties could make for a much more beautiful gemstone than the pure crystal the organisms depend on, which would explain why people prefer it over.

This isn't biologically implausible, lots of organisms use crystalline materials, or more often layers of crystal and tissue that is superior to the material on its own, including some in their eyes.

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    $\begingroup$ This is exactly what I was looking for. Thank you! $\endgroup$
    – Inbar Rose
    Commented Jan 4, 2017 at 13:31
  • $\begingroup$ I've added a picture to help your text. Roll back if it's undesired. I believe you've swapped the ordering (assuming it's ordered) of the iris and lens. Can you provide the proper name for the "colored fluid-filled bubbles" that improve color vision? I'm not familiar with them. Also, there are primates with nictitating membranes, like the lemur. Though, I don't see how suck a membrane could be replaced by a rigid crystal, can you expand on that? $\endgroup$
    – Samuel
    Commented Jan 4, 2017 at 17:43
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    $\begingroup$ One downside of crystalline eyes is that it adds one more dependency to the creature's dietary needs. That is, unless they can synthesize the materials from something common. Actually growing the eyeballs will be a challenge, too, not to mention you'd probably want to grow them to full size before wrapping a retina around. $\endgroup$
    – John Dvorak
    Commented Jan 4, 2017 at 17:48
  • $\begingroup$ @Samuel: I said the iris is "between" the cornea and lens, which is correct and matches your pictures. I don't know a proper name for the bubbles, even scientific articles I read just call them "oil droplets". There is a wikipedia article on them: en.wikipedia.org/wiki/Oil_droplet . The nictitating membrane is already pretty thick and rigid in many animals, so that wouldn't be a big change. $\endgroup$
    – TheBlackCat
    Commented Jan 5, 2017 at 0:15
  • $\begingroup$ @JanDvorak: The issue with the dietary needs is already addressed in the original question. Growth wouldn't be a huge issue, the lens could just grow in layers while the cornea could thicken and have material added to the edges. Seashells do that. The biggest issue would be tapetum lucidum, which isn't even a required feature. That could be handled by having it in pieces that and fuse in adulthood, or by simply having a flat retina. The round retina actually hurts the eye's optics, but helps the eye move more easily. But you could have an eye where only the cornea and lens move. $\endgroup$
    – TheBlackCat
    Commented Jan 5, 2017 at 0:21
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The Trilobite, an invertebrate from the Palaeozoic era had compound eyes that had lenses made of calcite crystal. http://www.sciencemag.org/news/2013/03/looking-trilobite-eye

These are quite often found in the fossil, record as intact structures as they are hard parts unlike other types of eyes made of soft tissue.

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Assuming you mean gemstones on top of the actual eye, like a protective layer:

Normally, pearls grow like stalactites/stalagmites, in layers of crystal, rather than a single crystal. At the boundaries of each layer, the fit is not perfect, so light is reflected and refracted at those points. The result: a translucent structure through which the organism can make out light and shadow, but not SEE, as such.

In order to get a working eye, the organism would need to have a layer of plasma in contact with the back of the gem, saturated with the mineral the gem is composed of. The back of the plasma layer would be a transparent organic sheath or cornea, allowing light to enter the eye proper. Ideally, the plasma would prevent crystal boundaries forming and the gem would grow uniformly, gradually getting pushed out as it grows. In practice, this is unlikely, so you'll end up with flaws in the gem, that inhibit sight.

The other issue is that gems, like other inorganics, dissolve only in ionic solvents, not covalent ones. I.e., the gem may be water soluble, but definitely not soluble in any organic solvent. If it is water soluble, then it will dissolve in rain, sweat, high humidity or any other environmental water. While this may not melt the whole gem, it will cause it to run, which will bend the surface in unpredictable ways, distorting the light entering the eye. If it is highly insolvent in water, (everything dissolves in water, to some extent) then it will take forever to build up a large enough concentration for the gem to grow to a visible size: the minerals will have to be absorbed by the body, once consumed; then transferred in dissolved form to the circulatory system (let's say bloodstream); since it won't dissolve in water or any organic compound, neither of these processes occur at any significant rate. It will therefore take years of a very fast circulation to get sufficient mass into solution. On the plus side, deposition will be quick; on the minus side deposition can occur anywhere, not just the eyes.

A possible alternate, would be a binary system: compound X is absorbed by the bloodstream, compound Y secreted by the cornea. When X comes in contact with Y, it precipitates compound Z and the rest remains in solution, to be excreted through the bloodstream. This process is the same as layering described above, with the additional problem of multiple crystals forming, making the gem useless as a lens.

TL:DR, probably not going to work well

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  • $\begingroup$ Wow, very useful. I was hoping for just this kind of technical answer - but I was hoping for one that would support my idea. Do you think given that it's a fantasy world, and such creatures could theoretically live a very long time that a substantially sized gemstone (say the size of a snowpea) would be possible? And if so, how long would that take given different constraints? $\endgroup$
    – Inbar Rose
    Commented Jan 4, 2017 at 11:22
  • $\begingroup$ @InbarRose The only reliable method we have of creating large crystals is by taking many small crystals and applying high temperature and pressure to fuse them together; even then they need to be cut and polished to get rid of rough edges. In a fantasy setting, you could put the organism is a corrosive setting that scours away the outermost part of the gem, creating a polished effect. Similarly, corrosive chemicals behind the gem could smooth the inner side, but that would prevent growth of the crystal/gem $\endgroup$
    – nzaman
    Commented Jan 4, 2017 at 14:49
  • $\begingroup$ @nzaman: There are ways to bind minerals (and other atoms) to proteins that allows much more to be carried in the bloodstream than could be carried in a water solution, and allows them to be moved through non-polar areas like cell membranes. It is also possible to change the local chemical environment to control the direction of chemical reactions to shrink crystals in one area and grow them in another. Organisms already build up crystalline structures using ionic components using approaches like that. $\endgroup$
    – TheBlackCat
    Commented Jan 5, 2017 at 0:44
  • $\begingroup$ @TheBlackCat But would the resultant be transparent, or a collection of overlapping grain boundaries, e.g., bone, coral? I confess the majority of my knowledge of crystallization techniques is of industrial processes, not biological ones $\endgroup$
    – nzaman
    Commented Jan 5, 2017 at 6:38
  • $\begingroup$ @nzaman: It depends on the material and how it is structured. A transparent material laid down in thin layers, or more likely alternating thin layers of material and protein, would still be transparent. Lots of fish have transparent bones right now. $\endgroup$
    – TheBlackCat
    Commented Jan 5, 2017 at 12:58
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You want the eye to be something that (suitably cured and preserved) is a beautiful and durable artifact.

Consider some type of compound eye. The surface can appear to be a diffraction grating, and might even show some kind of hologram effects, which could up the price if patterns appear that are considered auspicious.

They might have a natural covering that is as hard as glass.

They are somewhat convex shapes, but the eye itself is a thin shell. It would be applied to another object thanks to its thinness (like mother of pearl) or be prepared around a shaped solid form, perhaps a section of skull from the same creature.

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This is probably not what you wanted but it could be useful for a different world building.

Maybe these creatures latterly just have crystal material build up in their eyes. . It is not a beneficial trait, but a trait they learn to live with. As infants they have good vision to help protect them as they learn to walk and what food is good to eat. In adolescence their vision starts to fade as the crystals start to form in their eyes. (Rhino’s have poor vision but can still function) As full grown adults they are blind and now rely fully on their sense of smell and hearing.

If someone were to harvest an eye they would notice that they resemble geodes

Maybe these creatures constantly grow new eyes as they age. If this were the case you could get a rough idea of how old they were by the number of crystallized eyes they have.

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  • $\begingroup$ So, eyes would form, and then crystallize, like limbs on a gecko - interesting idea. $\endgroup$
    – Inbar Rose
    Commented Jan 5, 2017 at 9:56

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