Zebra Rock: One of The Rarest Rocks on Earth

The Story of Western Australia's Unique Zebra Rock

Six hundred million years ago, the Kimberley region was a very different place. The world had just emerged from a brutal ice age, glaciers retreated across the land and left behind a landscape of shallow lakes, muddy floodplains, and quiet lagoons. Into these waters, season after season, fine clays and silts drifted down and settled to the bottom. Imagine a still, muddy lakebed, layer upon layer building up over time. Slowly, this became the Johnny Cake Shale — a flat, featureless mass of mudstone and siltstone. Nothing about it would have caught your eye.

*Image depicts Zebra Rock specimen in the study: Mineralogy and geochemistry of pattern formation in zebra rock from the East Kimberley, Australia

But hidden inside that mud were tiny grains of pyrite — fool’s gold. These little crystals would turn out to be the key.

For ages, the mud sat near the surface, drying out and cracking in the dry season, soaking and swelling in the wet. Then, at some point, oxygen-rich groundwater began to trickle through. Maybe it was just rainwater. Maybe it was acidic fluid tied to some volcanic activity in the region. Geologists still argue about the exact trigger. What they agree on is this: when that water hit the pyrite in the mudstone, the chemistry of the whole rock changed.

Pyrite began to rust. As it oxidized, it released iron and sulphate into the groundwater and created acid. Each little pyrite grain was like a tiny acid factory, pumping out sulphuric acid into the surrounding clay. The rock was effectively pickled. Minerals started to dissolve, new ones began to form, and the chemistry of the mud turned into a cauldron of reactions.

If you want a visual way to understand this story, scroll down to the bottom to see the video we made on this on the OzGeology YouTube channel.

The Chemical Reactions That Made Zebra Rock

The first casualties were minerals like sericite, a fine-grained mica. Acid stripped away its aluminium and potassium, transforming it into a new mineral called alunite, a potassium-aluminium sulphate. Kaolinite, the common clay in the mudstone, was attacked as well, leached of some of its aluminium. What was left behind was an odd mix: quartz grains, alunite, and chemically altered clays.

Now here’s the part that really matters for the stripes. All that iron released from pyrite was sloshing around in acidic groundwater. Under the right conditions, it oxidized further and dropped out as hematite, an iron oxide. But instead of staining the rock evenly, it did something far more interesting. The iron formed in precise, rhythmic bands. This is what geologists call Liesegang patterning — when chemical reactions self-organize into repeating bands because of diffusion and periodic precipitation.

*Image Depicts Liesegang Patterning

Most Liesegang patterns are seen in chemistry labs or in small-scale mineral deposits — rings inside agates, bands in cave deposits, even rust streaks in concrete. To see them preserved on such a large, visually striking scale in a rock like Zebra Rock is extraordinary. It tells us that the conditions weren’t just fleeting but stable enough to let the reaction fronts move through large bodies of sediment without being disrupted. This helps explain why Zebra Rock is unique worldwide: it’s not just the chemistry; it’s the stability of the environment over long spans of time.

But in the case of Zebra Rock, it works like this: iron-rich fluid diffuses through the rock. Nothing happens for a while. Then, when the concentration of iron and the acidity reach a critical point, hematite suddenly precipitates in a line, using up the local iron. Then there’s a pause until enough builds up again to trigger the next band. Stripe after stripe forms, each a little distance from the last. The result is the regular, ruler-straight bands of Zebra Rock.

This wasn’t a slow, layer-by-layer deposit from water currents, as early geologists thought. These stripes cut across the original bedding. They are the frozen trace of chemical fronts sweeping through the rock, laying down pigment like brushstrokes.

*Image depicts an example of the hematite banding.

And the pattern didn’t always come out the same. In some places, the hematite formed spots or rods instead of stripes, depending on how the reaction fronts moved. In others, you can see faint branching or merging of bands. But the effect is always mesmerizing: pale bands bleached of iron alternating with deep red stripes of hematite.

While hematite was painting stripes, the leftover chemistry was still busy. As the acid began to neutralize, aluminium and silica that had been leached from clays started recombining. They crystallized into a mineral called dickite — essentially a more ordered form of kaolinite. Yes, the name always raises eyebrows, but geologists take it seriously. The presence of dickite tells us this wasn’t just surface weathering. It needed long timescales or slightly warmer conditions during shallow burial to form.

So the mineralogy of Zebra Rock reads like a chemical diary. First came intense acid: pyrite to hematite, sericite to alunite, kaolinite partly consumed. Then came neutralization: iron locked in as hematite stripes, aluminium and silica re-precipitating as dickite, and excess silica forming tiny quartz crystals. Even unusual minerals like svanbergite, woodhouseite, and florencite pop up — rare phosphates that only form under acidic, oxidizing conditions. Together, they confirm this all happened near the surface, in an environment open to oxygen, not deep underground.

Dating The Formation of the Patterns

And we know the patterns formed early, while the sediments were still relatively soft. Why? Because little pyrite molds and reduction spots inside the light bands are slightly flattened, squashed during later compaction. That tells us the banding was there before the rock was buried deep and compacted by overlying sediments. If the patterns had formed later, we wouldn’t see them deformed like that.

But Zebra Rock didn’t just sit there untouched forever. Not long after, in the Cambrian, massive lava flows from the Antrim Plateau volcanics poured over the region, entombing the Ranford Formation — and with it, Zebra Rock.

The Flood Basalt Event That Sealed Away Zebra Rock

Around 511 million years ago, vast fissures tore open the crust and unleashed the Kalkarindji large igneous province. This was no ordinary eruption. It was a flood of fire: basalt lava pouring across northern Australia, covering an area greater than a million square kilometres. The Antrim Plateau Volcanics, as this local chapter is called, spread like a molten sea across the land.

*Extent of the Kalkarindji Large Igneous Province

For the rocks of the Ranford Formation, including the lenses of Zebra Rock, this was catastrophic burial. Imagine being entombed under hundreds of metres of lava — the weight, the heat, the sheer immensity of it. For many rocks, that would have been the end of their story. Minerals would recrystallize, delicate structures would vanish, and any trace of subtle geochemistry would be obliterated.

But Zebra Rock endured. Why? The temperatures were high, but not too high. Estimates suggest the Ranford Formation was buried to depths of 3–4 kilometres during this time, with conditions hot enough to alter but not to destroy. The clay minerals and iron oxides survived, their delicate banding intact. In a way, the basalt flows acted as a lid, sealing Zebra Rock away from further surface weathering and protecting its patterns.

Only relatively recently, geologically speaking, did erosion and uplift bring Zebra Rock back to the surface. Today, it can be found in scattered outcrops along the Ord River and Lake Argyle. Miners and collectors prize it, slicing it into ornaments and sculptures that show off those hypnotic stripes.

Zebra Rock: A Unique Specimen Found Only in Western Australia

So, what makes Zebra Rock unique? Lots of rocks have bands, from banded iron formations to striped sandstones. But nowhere else do we see this exact combination: fine mud laid down in an ancient lake, laced with pyrite, infiltrated by acidic oxygenated fluids, and transformed by self-organizing chemical reactions. Add the perfect preservation conditions, and you’ve got a geological masterpiece that’s truly one of a kind.

Zebra Rock is more than just pretty stripes. It’s a time capsule. It records the chemistry of an Ediacaran wetland — the world just before complex animals took off. It shows how rocks can self-organize into patterns as precise as a painting.

That’s the story of Zebra Rock: born in mud, painted by chemistry, preserved through fire and time, and revealed again to dazzle us half a billion years later.

Here's the video we made on this on the OzGeology YouTube Channel. It will give you a far better idea of the geochemistry that formed Zebra Rock.