A harder form of diamond that has eluded scientists for decades can now be synthesised in the laboratory, and could be used to make extremely tough cutting and drilling tools.

Diamonds as we know them have a cubic arrangement of atoms in their crystalline structure. But for at least 60 years, we have been aware of another form – hexagonal diamond – that is much tougher, thanks to its crystals having no uniform shear lines along which breaks can propagate.

Natural hexagonal diamond occurs in meteorites, where it is known by the mineral name lonsdaleite, but only in mixtures with cubic diamond. Previous attempts to synthesise hexagonal diamonds have yielded only tiny traces that are similarly impure.

Now, at the Center for High Pressure Science and Technology Advanced Research in Beijing and his colleagues have succeeded in creating a relatively large sample of hexagonal diamond that is 1 millimetre in diameter and 70 micrometres thick, with purity close to 100 per cent.

While normal diamond has been synthesised for some time, the researchers explored a range of pressures and temperatures to find a sweet spot in which hexagonal diamonds were produced. This ended up being 1400°C at 20 gigapascals – 200,000 times the atmospheric pressure on Earth.

Such a material has never been made before, so it will need to be thoroughly studied to determine its properties, says Mao. “It’s incredibly valuable,” he says. “But once we know how to make it, anyone can produce it. So then the important thing is to get a patent and find a way to make it less expensive.”

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Hexagonal diamonds are predicted to be about 60 per cent harder than regular diamonds based on their structure. Cubic diamond has a hardness of around 115 gigapascals when measured in a Vickers hardness test. The hexagonal diamond created by Mao and his team measures 120 gigapascals, but they believe they can improve this significantly as they develop their technique further.

If hexagonal diamond can be synthesised with sufficient thicknesses, it could be used to make harder and more resilient tools for a range of uses in industry, such as drilling for geothermal energy, says at the University of Cambridge. “Obviously, the deeper you go, the hotter it gets, [and] it could enable them to go deeper underground.”

The famous marketing slogan about how a diamond is forever may only be a slight exaggeration for a diamond-based system capable of storing information for millions of years – and now researchers have created one with a record-breaking storage density of 1.85 terabytes per cubic centimetre.

Previous techniques have also used laser pulses to encode data into diamonds, but the higher storage density afforded by the new method means a diamond optical disc with the same volume as a standard Blu-ray could store approximately 100 terabytes of data – the equivalent of about 2000 Blu-rays – while lasting far longer than a typical Blu-ray’s lifetime of just a few decades.

“Once the internal data storage structures are stabilised using our technology, diamond can achieve extraordinary longevity – data retention for millions of years at room temperature – without requiring any maintenance,” says at the University of Science and Technology of China in Hefei.

Wang and his colleagues worked with small pieces of diamond only a few millimetres long, although they say future versions of the system could be in the form of larger storage discs. Their method used ultrafast laser pulses to knock some of a diamond’s carbon atoms out of place, leaving behind empty spaces the size of single atoms that each exhibited a stable brightness level.

By controlling the energy of the laser, the researchers could make multiple empty spaces at specific sites within the diamond, and the density of those spaces influenced each site’s overall brightness. “The number of empty spaces can be determined by looking at the brightness, which allows us to read the stored information,” says Wang.

The team then stored images – including Eadweard Muybridge’s 1878 sequence of photos showing a rider on a galloping horse – by mapping the brightness of each pixel to the brightness levels of specific sites inside the diamond. The system saved this data with more than 99 per cent accuracy and completeness.

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This storage method isn’t yet commercially viable because it requires expensive lasers and high-speed fluorescence imaging cameras, along with other devices, says Wang. But he and his colleagues expect that their diamond-based system could eventually be miniaturised to fit within a space the size of a microwave oven.

“In the short term, government agencies, research institutes and libraries focused on archiving and data preservation would likely be eager to adopt this technology,” he says.

The skies of icy planets across the cosmos may be full of diamonds. Compressed carbon compounds can turn into diamonds at less extreme temperatures than researchers thought were required, which may make diamond rain a common phenomenon inside ice giants.

In the past, laboratory experiments have led to confusion about the conditions under which diamonds could form inside ice giants such as Uranus and Neptune. There are two types of experiments investigating this: dynamic compression experiments, in which carbon compounds are subjected to a sudden shock, and static compression experiments, in which they are placed inside a chamber and compressed gradually. So far, dynamic compression experiments have required much higher temperatures and pressures to form diamonds.

at the SLAC National Accelerator Laboratory in California and his colleagues performed a new set of experiments using static compression but dynamic heating, compressing polystyrene – the same polymer used to make Styrofoam – by squeezing it between two diamonds and then hitting it with pulses of X-ray light. They observed diamonds beginning to form from the polystyrene at temperatures of about 2200°C and pressures around 19 gigapascals, conditions similar to those in the shallow interiors of Uranus and Neptune.

These pressures are much lower than the pressures found to be necessary for diamond formation in earlier experiments using dynamic compression. The reaction took longer than dynamic compression experiments typically run, which might explain why such experiments haven’t picked up low-pressure diamond formation. “It disagreed with established results and wasn’t what we expected to see, but it fit in nicely and sort of tied everything together,” says Frost. “It turns out that was all down to different timescales.”

This could mean that diamond rain is possible on smaller planets than we previously thought. Of the 5600 or so confirmed exoplanets, the researchers calculated that more than 1900 could have diamond rain.

It also means that within the solar system, diamonds could form at shallower depths than we thought, which could change our understanding of the dynamics of the interiors of giant planets. This shallower formation could allow the diamond rain to pass through a layer of ice as it sinks towards the centres of these planets. This would, in turn, affect the icy worlds’ magnetic fields, which are complex and poorly understood.