While most of us judge diamonds by their appearance, it's what's inside these sparklers that you can't see that makes them so valuable to science.

Key points: Scientists have long suspected there's a reservoir of primordial rock somewhere in the Earth's mantle

Scientists have long suspected there's a reservoir of primordial rock somewhere in the Earth's mantle A study using super deep diamonds has provided the first direct evidence

A study using super deep diamonds has provided the first direct evidence Diamonds give us a way to look directly at what's happening in the deep Earth, hundreds of kilometres beneath our feet

Microscopic bubbles of helium trapped within the stones have provided the first direct evidence of a reservoir of primordial rock deep below our feet that has remained relatively undisturbed since our planet formed roughly 4.5 billion years ago.

And that finding, published in the journal Science today, could lead us to a greater understanding of how the Earth works, and how it's evolved over time.

"For scientists, diamonds are not just the shiny jewels that they are for the rest of the people," said geochemist Suzette Timmerman, who conducted the research as part of her PhD at the Australian National University.

"For us, they're the direct window into the deep Earth."

Unravelling a mystery

Today the Earth is composed of an inner solid core, a liquid outer core, the mantle and crust. ( Getty Images: DEA / D'Arco Editori )

The Earth is basically split into four main layers: the inner solid core of the planet, the liquid outer core, the mantle and the crust.

The mantle, which is the thickest layer at approximately 2,900 kilometres, is divided into an upper layer, where tectonic movement takes place, and a lower layer.

It's long been thought there's a reservoir of primordial rock as old as the Moon, and potentially as old as the Earth, sitting somewhere in the mantle.

Scientists first suspected this primordial reservoir might exist in the 1980s when they discovered some basalt lavas had an unusual chemical signature.

Basalts spewed out of island volcanoes such as those found on Hawaii or Iceland had a very high helium 3 to helium 4 isotope ratio.

Unusual chemical signatures have been found in basalt lavas in places like Hawaii. ( Pexels CC: Brent Keane )

But this pattern was not seen in basalt that erupted from mid ocean ridges as the seafloor spreads apart.

"People have not known where these [unusual] basalts come from," said geologist Stephen Foley of Macquarie University who was not involved in the study.

"There have been helium isotope difference between ocean islands and between mid-ocean ridges, and really we don't know why."

This is significant because the amount of different helium isotopes present tells you where the helium came from.

"Helium 3 is there from the beginning of the Earth, you can't form it again," Professor Foley said.

While helium 3 can only come from a primordial source, helium 4 can form from radioactive decay of other elements, for example uranium and thorium.

But while the helium signatures pointed towards the existence of a primordial reservoir, they could not prove it actually existed.

That's because as magmas make their way through the mantle to form basalt lavas at the surface, their composition can change as they interact with the surrounding material.

There was even debate as to whether such a reservoir could have survived at all, particularly given the chaotic early history of our planet.

"When the Earth formed it was really, really hot, so we wouldn't have had a crust or an ocean like we know it today," Dr Timmerman said.

"In addition, we had a lot of meteorite impacts, and we had this big impact when the Moon formed, part of the Earth was just basically smashed away."

To crack the mystery of the primordial reservoir, scientists needed to study something else: diamonds.

Enter super deep diamonds

Material trapped inside diamonds can tell us a lot about what the conditions were like when they formed. ( Supplied: Antony Burnham )

Diamonds are created under great heat and pressure deep within the Earth's mantle and are delivered to the surface by volcanic activity.

The beauty of diamonds is they don't change on their journey.

And from the tiny bubbles of minerals trapped inside them we know at what pressure and temperature they were formed.

"Diamonds are a little bottle, as soon as you've formed it then you've encapsulated that fluid and it's not going to change," Professor Foley said.

"If you can measure that fluid then you've got exactly the sample that was there at the time the diamond formed."

Dr Timmerman's team didn't just look at any diamonds, they looked at super-deep diamonds sourced from Brazil.

Dr Timmerman said while this study using super deep diamonds was super cool, there are many things we still need to find out about primordial reservoirs. ( Supplied: Gareth Davies )

These rare rocks come from 410 to 660 kilometres below the ground, more than twice the normal depths you would find diamonds, from a part of the Earth's mantle known as the transition zone, which divides the upper mantle from the lower mantle.

"This is a way to directly look into the deep Earth without having any change when it was brought up to the surface," Dr Timmerman said.

The team first cut 24 diamonds into slices and looked at their growth structures under an electron microscope.

Then they analysed what minerals, trace elements and isotopes had become trapped inside the diamonds as they formed.

It's the first time such an analysis has been carried out.

They discovered the diamonds had high helium 3 to helium 4 ratios, confirming the primordial reservoir exists either at the depth at which the diamonds were formed or below them in the lower mantle.

Professor Foley said the study clears up an age-old controversy, and shows how crucial diamonds are as the only samples we have to give us information about the deep Earth.

"These super deep diamonds, and studies of them, are telling us what's really going on at depths of 400 and 500 kilometres," he said.

The diamonds also contained evidence of surface sediments, showing that material from the Earth's crust is being drawn deep into the mantle and mixing with these other materials.

Dr Timmerman said the discovery gave us tantalising clues about the movement of heat and material through the mantle, as hot material rises towards the crust and cold material sinks back towards the core, a process called convection.

"We know the mantle is convecting, but this reservoir hasn't taken part in that so it means we don't have convection in the entire mantle," she said.

Electron microscope images can reveal the different growth layers of diamonds. ( Supplied: Suzette Timmerman )

While Dr Timmerman was excited by what they had found so far, there are many other mysteries deep beneath our feet to solve.

"The questions that we still want to answer, how many of these primordial reservoirs are there down there, how big are they, and to know a bit more about the chemical composition and how they form," she said.

"It's really important to know the structure of the Earth and the volumes and the ages of these different chemical reservoirs and their composition, to understand how the Earth has evolved into a core and mantle, and how this has all been changing over time."