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I have a cave that is ravine like and stretches approximately one kilometer down into the Earth. Let's assume that the cave has a clear, crystalline roof allowing some light in.

Is there any sort of crystal, mineral, substance, or something else naturally occurring that could reflect light into most parts of the cave?

I want the cave to have enough available light for average plants and such to grow with the upper half of the ravine.

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  • $\begingroup$ Do the naturally occurring materials need to be left as-is, or can people modify them to make better tools, like mirrors? $\endgroup$ – Dan Dec 24 '18 at 16:15
  • $\begingroup$ Also, if the materials can be reworked, what level of technology are we talking about? $\endgroup$ – Dan Dec 24 '18 at 16:16
  • $\begingroup$ The materials must be as is and can't be reworked. $\endgroup$ – Thalassan Dec 24 '18 at 20:19
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Ice.

ice cave

https://www.getyourguide.com/joekulsarlon-l2030/crystal-ice-cave-tour-from-jokulsarlon-t73050/

The Icelandic ice caves (in glaciers) are made of translucent ice which admits sunlight from above. The light bounces around, refracting within the ice, reflecting off of other ice surfaces within the cave and illuminating the interior to a degree you would never see in a regular stone cave.

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    $\begingroup$ 100K! Congratulations! Also, does this meet the OP's requirement that plants can grow? Less light is needed to illuminate than to sustain life. $\endgroup$ – JBH Dec 24 '18 at 5:11
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    $\begingroup$ @JBH We would require more info on the levels of light experienced in the caves, but there are many plants that can thrive on minimal light (even some that start to die if they get too much!) - some examples can be found here, but I notice that a lot of them have a minimum temperature to survive - perhaps a hot spring in the cave? $\endgroup$ – Chronocidal Dec 24 '18 at 8:24
  • $\begingroup$ But would ice under constant sunlight melt over long periods of time? Would the flora be flooded with water from time to time? Would the ice move like it would in a glacier? The ice cave in the link while beautiful doesn't seem to support plantlife. $\endgroup$ – Neil Dec 24 '18 at 8:36
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    $\begingroup$ Thanks for the comments. Ice caves have moss and that is about it - it is cold and I am sure the soil is poor. But you can definitely have photosynthetic communities in bodies of water under a layer of ice. Re sunlight melting the ice - not if it is super cold! Glaciers can be very sunny. I envision this as a layer of ice insulating the warm (geothermal?) ravine below, with subzero air above. Water conducts heat well. Condensation from below freezes onto the layer and replenishes it, compensating for losses to sublimation on the top. $\endgroup$ – Willk Dec 24 '18 at 17:51
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Unfortunately there is limited chance that enough light could make it through the crystaline roof even in perfect conditions to allow for plant growth. part of this is indeed the type of material the roof is made off, but also the rotation of the earth, and a few other minor factors.

Light, Mirrors and Crystals

Although there are many materials that are translucent enough to allow light through, as L.Dutch explained these need to be polished to allow that to happen. even if you had the walls of the cave covered with nice reflective surfaces made of Gold or Silver, each metre that was reflective means that same metre cannot have life growing on it. and again each of those metres that do have life means less and less life is growing as there is less light bouncing around.

As WillK suggested, Ice is a potential option and doesn't need to be polished. however... this still has the same issue with any surface allowing reflection means a surface not housing life, and even then, as far as i'm aware, a wall of ice (insert random game of thrones reference) is not really suitable base for growing life.

Earth's Rotation

Even in optimal conditions light would only reach half way down a one kilometre ravine for less than a hour a day. That's not a lot of time for plants to absorb sunlight for photosynthesis, then consider a lot less light is making its way through the roof material, and on a cloudy day that is diminished even more, and each surface the light bounces off diminishes it even further etc, so plant life would really really struggle to photosynthesize...

Why Photosynthesize?

Ignoring fish for a minute... there are plenty of examples of life at the depths of the oceans, and corals that don't need light to live. Many of these live around hydro-thermal vents, and they get their energy needs from converting the abundance of nutrients and minerals around them, this process is called chemosynthesis, this happens at a bacterial level and then other plants and corals live off those bacteria.

Its very plausible that given the right geothermal conditions that life would exist in these caves in pools of water... perhaps those pools come from the melted glacial ice that forms the roof

Mix this with Fungi that don't photosynthesize but live off their environment, which could include living off the chemosynthesizing life... and you have most of a proper life cycle right there.

https://en.wikipedia.org/wiki/Chemosynthesis

Then of course there is the very theorectical Thermosynthesis, although this has never been seen before in real life.

https://en.wikipedia.org/wiki/Thermosynthesis

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Is there any sort of crystal, mineral, substance, or something else naturally occurring that could reflect light into most parts of the cave?

I am afraid not. While it can happen that some naturally occurring materials are transparent (think gems), to have reflection you need to have both the right material AND the right surface condition, meaning a polished surface.

While some metals can be worked to make a pretty decent mirror, they never naturally come in nicely polished surface state. They shine, but don't reflect like a mirror.

Think of gold or silver nuggets, since they are the only two naturally occurring metals.

gold nuggets

You can still have reflective surfaces in the right conditions, like grazing light on water surface, but that would

  • complicate the internal design of your caves to have the light traveling deep into the cave
  • work only in precise moments of the day, since the external light source won't be stationary in the sky
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  • $\begingroup$ Thanks for your Input! Any possible way I could explain naturally occuring silver and gold deposits that are polished. Whether it be through natural chemical or physical means. $\endgroup$ – Thalassan Dec 24 '18 at 6:15
  • $\begingroup$ Maybe if your setting is a desert-like place you could explain it by frequent sandstorms? The nuggets would be sandblasted every now and then to keep them polished. $\endgroup$ – Esben Boye-Jacobsen Dec 24 '18 at 13:20
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    $\begingroup$ @EsbenBoye-Jacobsen, sandblasted surfaces are not polished. $\endgroup$ – L.Dutch Dec 24 '18 at 13:56
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I think your best bet is a quartz cavern that has collapsed.

https://i1131.photobucket.com/albums/m550/SSgtMatt/crystal-cave-4.jpg

http://chantcrystalhealing.co.za/crystal-caves/

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