I had a dream that on my alien planet, there was a large desert that had naturally occurring glass marbles in the sand. My dream logic was that the sand was melted into glass and natural weathering turned the glass into a spherical shape.

Obviously a habitable planet would not be able to harbor such extreme weather, and I realized that immediately when I woke up. So, my theory is the only way to do this on a habitable planet is through lightning strikes on sand + gritty wind and sand weathering to turn it into a sort of glass pebble.

Is this feasible? What is another solution if not, while still leaving the planet habitable for large terrestrial lifeforms? (they don't live in the desert but the rest of the planet needs to be survivable)

A few details about the planet:

  • It is larger than earth but composed of similar materials
  • The atmosphere has a higher percent composure of oxygen
  • The planet has larger oceans than earth and very few continents. (Think about a continent about twice the size of Australia. That is the largest landmass, and everything else is smaller islands)
  • The atmosphere is thicker than earth, so it is more insulating.
  • There is a very slight axial tilt, less than earth's.
  • Their moon is larger than Luna.

A few details about the desert:

  • There is volcanic activity in the desert
  • There are lightning strikes
  • It is close to the coast
  • It harbors dunes and the weather that accompanies that
  • On the flat parts of the desert and by the coast and rivers, there is some plant life.

What I want the marbles to look like:

  • Ultimately, I want them to be smooth but they don't need to be round. Ovular or flat are okay. The glass should be pretty thick as well, not incredibly brittle. It doesn't need to be transparent in it's natural state, but it should be able to be transparent when processed.
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    $\begingroup$ Waayyyy out thought : could some bizarre creature poop glass or a glass-like substance ? $\endgroup$ – StephenG Aug 7 at 17:11
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    $\begingroup$ @StephenG That would be a sufficient answer to the question if you can come up with a feasible biological process. $\endgroup$ – Tardigreat Aug 7 at 17:57
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    $\begingroup$ "Alas" squishy, smelly biological stuff is not my area of expertise. :-) $\endgroup$ – StephenG Aug 7 at 17:59
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    $\begingroup$ @StephenG Alas indeed hahaha $\endgroup$ – Tardigreat Aug 7 at 18:03
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    $\begingroup$ ObXKCD: xkcd.com/260 $\endgroup$ – IanF1 Aug 8 at 5:24

10 Answers 10


There are 3 types of natural glass: obsidian (volcanic glass), impactite (meteor glass), and fulgurite (lightning glass).

Naturally round impactite is virtually unheard of. Impactite tends to have strong striations in the direction of impact that gives it more of a cleaved shape. It is also very rare for it to be transparent. Technically, a round transparent piece of impactite might be possible, but probably not a whole field of the stuff.

Obsidian can be naturally round, but due to volcanic ash that is mixed into it as it's made, it's not transparent, even when polished.

Fulgurite is normally the most transparent type of natural glass because it is formed just from the sand that the lightning hits without being mixed up with impurities from ash, meteorite, or granite below the sand. That said, it's also the least round because it takes the shape that the lightning moved through the sand.

Any of the above could be made very round and polished in a river bed, but desert sand would erode them unevently.

It's pretty unlikely that any of these forms of glass would match your description, but perhaps a gemstone would. Your dream reminds me of a documentary I saw on desert diamonds a few years back. Because diamond is harder than sand, they don't errod into weird shapes like natural glass would.

enter image description here

They are naturally round and just sit around on the surface in certain areas of the deserts in Africa and the Middle East. Like most natural glasses & gemstones, uncut diamonds are typically translucent enough to see light through, but need to be polished to become transparent like glass. That said, uncut diamonds can sometimes have a very good natural clarity to them; so, it may be possible to find a desert strewn with diamonds much like the more round ones in the picture below.

enter image description here

The richest and most obvious of these diamond deserts have already been picked through, but there are (or at least were) places right here on Earth that very closely resemble your dream. You probably don't need to change the alien world to make this plausible, you just need to be in the right desert on it.

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    $\begingroup$ I chose this as the best answer because it elaborated about the other types of natural glass, while proposing something that was unexpected but still meets the question's criteria. $\endgroup$ – Tardigreat Aug 9 at 6:24

Your desert glass pebbles are volcanic glass. In the US they are called Apache Tears.

apache tears in a bucket source

These are unpolished, as found in the desert. When polished they are a very beautiful deep black and suitable for use in jewelry. Some of the round ones in this image ones look pretty close to that without polishing.


Apache tears" is the popular term for rounded pebbles of obsidian or "obsidianites" composed of black or dark-colored natural volcanic glass, usually of rhyolite composition and bearing conchoidal fracture. Also known by the lithologic term "marekanite", this variety of obsidian occurs as subrounded to subangular bodies up to about 2 inches in diameter, often bearing indented surfaces...

Volcanic glass forms naturally and naturally breaks into these chunks, which are weathered into roundish shapes. Once you start with natural glass you could have them weathered and be polished to the end result you want - perhaps tumbled along a wadi by periodic floods?


Bring in a desert-dwelling animal or insect. There can be two ways to do this.

One is have an animal/insect attracted to the glass itself.

Depending on what it's made of, there could be minerals in it the animal needs, but the rest of the glass is useless or even poison. So the animal breaks off the glass and swallows it, leeching out the appropriate substances, then pukes up the rest. It comes out in a lovely glass marble shape, like a clam with it's pearl. It can come out the other end, too, but that's a little less...appealing.

Insects are great for sheer number and being able to live practically anywhere, but animals can make larger or even variable sized ones. Though if you had several insects eating the glass chunks from the outside, they could leave behind "balls" of varying shapes and sizes.

The other is an animal makes the glass.

Same basic idea, but instead of being attracted to the glass itself, it's merely hunting for food. If the animal is something like a snake, it would have a hinged jaw that opens wide to grab it's prey. Since this is an alien world, we could posit that the prey is small, swift, and hides in the sand, and what's been best for this critter's survival (evolution-wise) is the hinge-jawed creature scooping up it's prey, sand and all. It then regurgitates the sand later, and what emerges are these beautiful glass (or glass-seeming but not true glass) balls.

In either of these scenarios, over time glass balls would have built up enormously. Even if people now harvest them, these animals continue to burrow beneath the surface, leaving their balls there, and the winds that shift the sand around can also bury them, sometimes deeply, only to unearth them again later. Over the course of centuries you could have quite the motherlode.

Either idea would need more hammering out, of course, but it could be a viable way to get the effect you want.

  • $\begingroup$ A biological process is about the only way to restore balls fast enough to have them build up. $\endgroup$ – John Aug 7 at 18:35
  • $\begingroup$ I reckon you could make a great addition here referencing chicken grit raising-happy-chickens.com/grit-and-oyster-shell.html $\endgroup$ – Jeff Aug 9 at 19:48

The glass balls are fossilized honeydew.

The volcanoes in the desert were originally islands. The aphids that lived there were huge due to insular gigantism. Now since they are insects, they don't fossilise as well. When tectonic activity cause the island to connect to the mainland, and then dried the ocean to one side of it, all that was left was the fossilised honeydew. Then the wind blew the fossilised balls and they spread through the desert.

As to why this honeydew fossilised: a small part (~1%) of the giant aphids had a mutation that caused their honeydew to contain light activated resins, which caused it to harden and become inedible to the ants that tended them. The ants would still tend those aphids, but would leave their excrements alone. That is why you can find so much fossilised insect excrement from so many different ages around the desert.

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    $\begingroup$ I had basically the same idea, but with amber. $\endgroup$ – Stig Hemmer Aug 8 at 7:21

If you don't insist they be glass, then sure. Cave pearls are little spheres of calcium salts that form due to successive periods of wet followed by drying. Each dry period deposits another layer. They can be quite regular shaped. The shape arises due to the regularity of growth, and due to the fact a sphere is the most mass per surface area you can get.

So if, instead of glass, it was some sedimentary process, you could get many spheres. Conceivably there are materials that could be produced this way that are transparent or translucent. I don't happen to know one, but it's possible.


The following steps, inspired by the process used for manufacturing lead bullets in the past, can determine the formation of spheroidal glassy agglomerates:

  • take a sandy surface
  • let a meteor hit that surface
  • the impact will melt and splash material (sand) around
  • during its ballistic trajectory, the molten material will solidify in free fall
  • the solid shape will resemble that of a drop or o sphere
  • let the material impact a soft surface (i.e. water)
  • over geological times the water body can dry off, turning into a desert
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    $\begingroup$ What you've described actually occurs in real life, and the resolidified droplets of initially-molten glass thereby produced are called tektites, which come in various shapes - including, yes, spheres (as well as ellipsoids, teardrops, dumbbells, and other typical shapes for drops of [ex]liquid). $\endgroup$ – Sean Aug 9 at 20:45

Sorry no, at least not as a purely, earthbound, geological process, the environment doesn't allow for the amount of multi-directional rolling you would need to round glass into a ball.

Glass yes, probably in the form of fulgerites like the Mauritanian example shown below: enter image description here

Smooth wind sculpted surfaces yes, the below is a purely wind polished piece of desert glass from Libya:

enter image description here

But spherical or even near spherical no sorry.

You might conceivably get away with a biological process, a creature that eats sand to extract surface nutrients/bacteria/algae on the grains and due to digestive deposition of waste silica cements the grains together in its dung.

Given your newly stated goal the desert glass linked above is a good fit, it comes in quite large sections, it is however thought to be the result of a meteor impact rather than lightening strikes.

  • $\begingroup$ Water is an effective way to polish them to spheres. Perhaps the desert is the bottom of a dried up lake or sea, or once contained a sea. Op mentions the sea is nearby. Any shoreline erosion would round any pebbles. The only difficulty I see here is getting the glass pebbles off the beach and into sand, as they'd naturally be located with similarly sized pebbles $\endgroup$ – Innovine Aug 7 at 17:24
  • $\begingroup$ @Innovine But they won't stay round in a desert environment with windblown sand. $\endgroup$ – Ash Aug 7 at 17:26
  • $\begingroup$ You don't need rolling to make something spherical - having a molten droplet solidify while in freefall also works. That's how lots of tektites - as suggested by @L.Dutch - are spherical (or ellipsoidal or dumbbell-shaped). $\endgroup$ – Sean Aug 9 at 20:59
  • $\begingroup$ @Sean Most of the tektites I've seen are actually teardrops with quite a pronounced tail. $\endgroup$ – Ash Aug 10 at 4:17

If temperature in the mantle of your planet is very high, one can have geysers erupting with quartz lava turned into vapor. It would then condensate in form of spherical droplets.


Impactite--desert glass, is actually not that rare or difficult to find, and they are quite transparent as well.

For your desert, because the atmosphere were significantly denser, there could by significant past bolide impact above that desert, of which explode d in an airburst that is much larger than that of anything on earth.

At a very high impact yield, the radiative heat from the fireball can melt the sand down, or if the sand were deep enough, shock melting from a large impact could send rapidly cooling molten debris flying out into a significant strewn field. Both process results in glass meteorites, or desert glass. And if there were a series of bolide events, or there is a significant large impact, you can have this glass literally scattered all across the desert.

Now add in an animal that uses gastroliths for digestion: the animal will find the glass and swallow it, as the impactite were the only pieces of solid material small enough to be swallowed by an animal, yet large enough to grind up the animal's feed. The animal retain the glass for a while, using it to grind down the hard vegetation it eats, wearing them down until the glass pieces were too rounded for it's purpose. the animal will then pass the now rounded glass marble and poop them out, which are easily cleaned when the excrement decomposes. (Or the animals could retain the rounded balls until they dies and decomposes, leaving behind a pile of marbles)

Now after a few thousand years, maybe all the glass were polished, maybe the herds of animals were still doing their jobs, but you now ends up with a desert full of perfectly transparent, rounded glass marbles.

  • $\begingroup$ Considering that the human-level sentient animals of this planet use gastroliths to chew their food, this is a very feasible and creative idea. $\endgroup$ – Tardigreat Aug 11 at 4:10

I think natural marble balls could become possible by long term accumulation of sand in very desert areas over many hundreds of years. The strong winds of deserts could make the sand grams accumulate at different rates making marble balls to form and have different shapes and sizes.


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