Most rocks that you find on the  ground look like they belong there.

They’re rough and irregular,  a little jagged and gritty. But occasionally, geologists find  rocks that look a little out of place.

Perfectly spherical. Weirdly heavy. Incongruously smooth.

Or maybe so ornate and delicate that they look like they had to have been carved, like this one here. These rocks look so strange that they have accumulated more than their fair share of folk tales and conspiracy theories But they also have something in common, aside from the supernatural explanations. Even though they’re made of different stuff, and look visually distinct from each other, the geological processes that  formed them are exactly the same.

So here’s how these  impossible-looking rocks formed, and what that can tell us about our planet, and even some other ones. [intro music] There are a few out-of-this-world  rocks we wanted to talk about today, and we’re gonna start with Klerksdorp spheres. Miners in the Ottosdal region of South Africa, near the town of Klerksdorp, had been digging up these strange  spherical objects for decades, and in the 1970s, people started to wonder what they really were. They ranged in size from just millimeters to up to 4 centimetres across, with a smooth surface and a metallic sheen, and in some cases, a series of  grooves around their middle.

Basically, they looked like mini cricket balls. But here’s the thing. The rocks they were embedded in  were more than 3 billion years old.

Which makes them a lot older than both cricket and the humans that invented it. Some people have even considered the possibility that these were artifacts  left behind by ancient aliens. And while we know that these  rocks aren’t extraterrestrial, we’ve also found some bewildering  rocks that are extraterrestrial.

Because they are on Mars. In 2004, the Martian rover Opportunity touched down on the Meridiani Planum, and within its first weeks on the  planet it spotted something unexpected. Huge numbers of tiny metallic spheres  were scattered all over the surface, each between 1.5 and 5 millimetres in diameter.

Like the Klerksdorp spheres,  they were smooth and metallic, and they seemed to shine with a bluish  hue against the red martian soil. These so-called Martian ‘blueberries’ caused a storm when their pictures were released. Again, they seemed too different, and too perfect, to have a purely physical explanation.

Some wondered if they had been created by modern or ancient Martian lifeforms. However, the story got a bit more complicated when we found out that sometimes, they form on Earth too. In the deserts of Utah, metallic spheres can be  found littering the surface near outcrops of red Jurassic age sandstone.

There are a lot of different sizes, from as small as a pea to as big as a grapefruit. The native Hopi people who  first lived in this part of Utah have referred to them as  marbles, and I can see why. According to legend, their spirit ancestors return to  Earth at night to play marbles, then leave the balls to reassure  their descendents that they’re happy.

These mysterious minerals are odd  because they’re perfectly round. But while these last ones may not be spherical, they’re just as bizarre. I’m talking about fairy stones.

These fascinating rocks come in  all kinds of colors and sizes, from the smallest balls to  boulders called devil marbles, and can be found in sedimentary  deposits all over the world. The Moroccan variety of fairy stones  are usually a soft, chalky white. They're just the right size to  fit in the palm of your hand, and they're built from smoothly overlapping lobes that give each one a unique shape.

Honestly, they look like they’d  be more at home in an art gallery than in a geology collection. And just like when we look  for shapes in the clouds, our pattern-matching brains are really good at finding shapes in these Like this one looks like a bug or  like a kodoma from princess mononoke This one looks like a mushroom or  like a sock or like a weird tree One Scottish legend describes how these  rocks are actually gifts from fairies, formed at night on tiny potter's wheels and left to bring good luck  to the mortals who found them. So we’ve got fairy stones, Hopi marbles, Martian Blueberries,  and Klerksdorp Spheres.

All of them have been given supernatural or alien-based explanations for how they formed. But looking closer, the evidence that they’re  natural is pretty concrete. Which is a pun, that you’ll  get it in just a second.

For one thing, they’re made of mineral combinations  that do occur naturally, even if you wouldn’t necessarily  expect to find them where they are. The marbles, blueberries and cricket balls are all made of iron minerals, and the fairy stones are  composed of calcium carbonate. Despite their smooth outer surface, they don’t show any tool marks or  any indication of being worked, and inside they either have  little to no structure, or are made of simple concentric layers.

And one clue is really the smoking  gun to these objects being natural: when they’re found within rocks, which they frequently are, the layers of the surrounding  rock continue through them. This means that, despite  how out of place they look, they must have formed at  the same time as the rock. And we call these rocks that formed within rocks when part of the sediment becomes  mineralised differently to the rest, concretions.

I told you it was a relevant pun It’s a pretty simple process, but one that gives rise to some of the  oddest bits of geology on our planet, or on any other. Concretions form almost  exclusively in sedimentary rocks, which started out as layers of loose sediment that settled out from the air  or water that put it there. But it takes a long time before  sediments are turned into rock.

In the meantime, fluids  can circulate through them, and it’s these fluids that are  the secret sauce for concretions. It starts with a tiny grain of something. It could be a scrap of organic matter, a shell or something destined to become a fossil.

Mineral-rich fluids that are  hanging around within the sediments happen upon this grain and use it as a nucleation point to start precipitating a solid crystal matrix. It’s just like when you’re making rock candy, you use tiny grains of sugar as a starting point for the big sugar crystals to grow. Except for this real rock candy, the growth is happening  inside the sediment itself.

The mineral matrix grows from the inside out, trapping sedimentary grains and layers. If nothing gets in its way, this omnidirectional growth will  create a sphere of solid crystal. The time it takes can vary quite a lot, but it might be just months or years.

Over a much longer period of time, the rest of the sediment then turns  to rock around the concretion, through normal rock forming processes  of squeezing, drying and heating. But because of their early  and uniform mineralisation, concretions are often harder than  the surrounding sedimentary rock, meaning they survive when the  rest of the rock erodes away, leaving them on the surface  to confuse passing humans. Since the recipe for a concretion  only calls for a fluid saturated with mineral ions and something  to kick off crystallization, these solid mineral lumps are  actually surprisingly common in the geological record.

So, far from raising questions  about mysterious objects, concretions actually offer an opportunity to learn more about the  conditions of their formation. The Klerksdorp spheres were created  inside layers of volcanic sediments. Three billion years ago, eruptions piled up ash over the landscape, and depressions in these ash  layers filled with water, creating an acidic lake system.

These lakes were inhabited by  primitive microbes which would die, sink to the lake bottom,  and get mixed with the ash. As the specks of organic matter began to decay, they changed the chemistry of  the waters surrounding them, causing iron sulphide, aka  pyrite, to start to precipitate. If allowed to grow freely,  pyrite forms cubic crystals, but this pyrite was growing inside the ash, around the organic matter, and so it made microcrystalline  spherical concretions.

And the grooves on the surfaces  of some of the solid balls are remnants of the original  ash layers that contained them. So no, these rocks weren’t  the work of ancient aliens, but they do owe their existence to  some of the earliest life on Earth. Scientists think that a similar process helped to create the Martian blueberries and their earthly counterparts too.

This hypothesis says that in the deep past, the Martian soil was covered  by acidic liquid water that soaked through layers of sediment. And a change in groundwater chemistry, possibly caused by interactions  with the atmosphere, triggered the growth of iron oxide  minerals around sediment grains. Both the Opportunity and Perseverance rovers have found the spheres embedded  in the rocks that formed them.

As a result, the blueberries  represent a crucial piece of evidence to suggest that the red planet’s dry dusty surface was once a very different, far wetter, place. Likewise, on Earth, the Hopi marbles may have helped scientists  piece together Martian history from hundreds of millions of kilometers away. These terrestrial analogs  may have helped us understand the extraterrestrial concretions’ formation far better than we would by robotically poking at them  from a very great distance.

Finally we have the fairy stones, specifically, fairy stones found in Morocco. They look different from the  metallic spherical concretions, but they’re formed in a similar way, just starting with different minerals. These are carbonate concretions, and they likely formed early on  in the process of sedimentation.

And while they may not be evidence of fairies, in Morocco, they could hold  evidence of ancient life. Concretions usually need a nucleus to form around, like how oysters form pearls  around a small irritant. One study into concretions  from southwestern Morocco specifically wanted to look at  how quickly that process happened, and if the nucleus that set  off the concretion-making could tell us about the  paleoenvironment at the time.

The layers they were most interested in formed during the Upper Cretaceous, which is a time when lots of cool stuff  was happening with living organisms. And as it turns out, the concretions themselves are pretty great at preserving some of those fossils! For instance, the researchers found that ammonite fossils were more abundant inside  concretions than outside of them.

They also found that these  concretions held microscopic fossils like diatoms and planktonic foraminifera, all things that would have been  common in the shallow ocean waters that covered the area at that time. They even found some that formed  around ancient fecal matter! The existence of the concretions  is essentially evidence that the area was teeming  with life back in the day.

If it wasn’t, there wouldn’t  have been enough organic carbon around to form them in the first place. So even if these Moroccan concretions  can’t prove the existence of fairies, they do help us understand the living  things that lived here at that time. You know, the ones that actually exist.

Sorry fairies, I don’t mean to be mean So even though people all over the world have speculated about their mystical origins, the evidence shows us that these  concretions are all-natural. And SciShow Rocks Box subscribers will receive their own geological work of art  in the form of a fairy stone from Morocco! Signs of fossil life not included.

Though if you scrape it on a microscope slide  I’m not saying you won’t find anything These subscriptions have been incredibly popular, and lucky for you, we have some spots open! Every box comes with an info card to teach you all about your new specimen. Head over to Complexly.store/rocks to learn more, and maybe sign up.

Or if you prefer to buy your rocks a la carte, we do have some of our favorite minerals from past months available  for individual sale as well, along with some cool accessories and merch. So check it all out at Complexly.store/rocks. Thank you for watching! [ outro ]