The Earth is a wondrous place, but some of the most stunning things sometimes come from off-world. Take, for example, pallasite meteorites – extra-terrestrial rocks that are strung through with gemstones, offering tantalizing glimpses into our Solar System’s deep history.

Oh, you haven’t heard of them? Well, you’re in for a treat.

What is a meteorite, technically?

Meteors, meteorites, comets, asteroids… there are so many names for the various space rocks out there, it can sometimes be tricky to understand what precisely we’re talking about. So let’s start by clearing things up a little.

It starts, generally, with asteroids: a small bit of rock or metal floating around in space. Of course, “small” is a relative term; some of these asteroids can be up to 1,000 kilometers (620 miles) across, or roughly twice the length of the Grand Canyon.

Meteoroids come next – these are much smaller than asteroids, only reaching up to about a meter (3 feet) across. Should this tiny rock get close enough to Earth that it enters the atmosphere, one of two things can happen: either it will burn up in the atmosphere, leaving bright trails of color streaming through the sky – those are meteors – or else some rock will be left over. In that case, the lump left on the ground after this cosmic journey is the meteorite.

The different types of meteorite

If you’ve ever found a meteorite in real life, chances are overwhelmingly high that it was what’s called a stony meteorite. Something like 94 percent of meteorites fall into this category, and it’s exactly what it sounds like: they’re rocks. Made of… rocks.

Okay, maybe that’s doing them a disservice. They’re not as boring as all that: most are what’s known as chondrites, which, “at over 4.5 billion years old […] are some of the most primitive and pristine rocks in the Solar System,” explains the Natural History Museum. 

“Chondrites are the material from which the Solar System formed. They have been little changed compared with rocks from larger planets, which have been subjected to geological activity,” it notes. “Chondrites can tell us a lot about how the Solar System formed.”

At the other end of the spectrum, there are the iron meteorites, which again, are fairly self-explanatory. They’re rich in iron – or, often, iron-nickel compounds, sometimes with traces of sulfide and carbide minerals.

“These are rare and rather special beasts,” says New Scientist. “They are thought to come only from asteroids that grew so large that their innards melted under the extreme pressure. Within this interior, iron, which is more dense than rock, would have gradually sunk to the center.” 

“In other words, these were asteroids that were on their way to becoming planets with a rocky crust and a molten iron core, much like the Earth is now,” it explains. “Many of these giant asteroids were later smashed to smithereens, spitting out chunks of rock and iron, some of which eventually cross our orbit and end up on Earth.”

But it’s the bit in between these two extremes where things get interesting – because that’s where we get the stony-iron meteorites (hey, nobody ever accused astronomers of being inventive namers). These meteorites are formed of roughly equal amounts of iron-nickel and rocky minerals – and the results can be pretty stunning.

What are pallasite meteorites?

Of all the meteorites, pallasites are perhaps the most beautiful. They “contain big, beautiful olive-green crystals – a form of magnesium-iron silicate called olivine – embedded entirely in metal,” explains the Natural History Museum. “Sometimes the olivine does not occur as a single crystal but as a cluster. Elsewhere it can create a pattern of veins through solid metal.”

They’re very rare: “with only about 300 pallasites out of more than 60,000 recognized meteorites, they are even rarer than diamonds,” points out the UK’s National Space Centre. But somehow, that’s not the most interesting thing about them. Pallasites are also the most mysterious type of meteorite, since even today, nobody’s quite sure how they’re formed.

“The formation of pallasites is a subject of debate among scientists,” explains the National Space Centre. “Pallasites come from differentiated asteroids, which are asteroids that have melted, allowing them to separate into a layered structure of core, mantle and crust. The established theory for many years was that pallasites originate at the boundary between the core and the mantle, where iron from the core was squeezed up into the olivine in the mantle.” 

If that’s true, pallasites could be a fascinating insight into how terrestrial planets, like our very own Earth, are formed. “If we cut our Earth in half, we would also see an iron and nickel core in the middle. Around that would be the rocky mantle, and above that you would get the solid crust which is what we walk around on,” explained Caroline Smith, Head of Earth Sciences Collections at the Natural History Museum. 

“We haven’t been able to drill down into the Earth’s core, but geologists, seismologists and other scientists can use pallasites as analogues for the composition and structure of our own Earth and get a good idea about its interior.”

But not all experts are convinced by this origin story: there are too many pallasite meteorites, they argue, and not enough olivine-rich meteoroids out there in space, for them all to have been formed in this way. Instead, they think collisions between larger and smaller asteroids are to blame: “During the bustling, crowded early Solar System, hit-and-run collisions could have completely disrupted the planetesimal,” Smith explained, “mixing the core and mantle materials together.”

The Imilac meteorite

Because they’re so rare and beautiful, it’s no surprise that pallasites are popular for collectors. But some really are extra remarkable – such as the Imilac meteorite, which, at over 4.5 billion years old, dates back almost to the very beginning of our Solar System.

“It is a slice from one of the world’s largest specimens of its kind,” explains the Natural History Museum. “It’s thought to have been part of a much larger meteor that weighed up to 1,000 kilograms [2,200 pounds] and exploded over the Atacama Desert in northern Chile, possibly in the fourteenth century.”

It must have been quite the thing to witness – but even now, we have the spoils to enjoy. And like all pallasites, the Imilac rock isn’t just beautiful, it’s also a vital piece of scientific evidence.

“Imilac is stunning,” Smith said, “but it was also chosen because many meteorite examples can deteriorate in Earth’s atmospheric conditions.” 

“Imilac is a very stable meteorite, so even though it’ll be on display, it will still be accessible for research purposes.”