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Messages from earth's molten heart

Natalie Angier Dec 19 2017, 0:24 IST
A ruby crystal from the mines of Myanmar's Mogok Valley. Researchers have linked ruby creation to collisions between continental land masses, proposing that rubies be renamed

A ruby crystal from the mines of Myanmar's Mogok Valley. Researchers have linked ruby creation to collisions between continental land masses, proposing that rubies be renamed

Around 1920, Justo Daza, an experienced mine worker, and Fritz Klein, a mining engineer, were scrambling over the steep mountainside terraces of Chivor, a legendary emerald site in northeast Colombia. They were breaking rocks apart with long iron poles and explosives packed into drill holes. They were hunting for new emerald veins and not finding any. Let's move on, Fritz said. This area is dead. No, no, no, Justo insisted. There's emerald here, I know it. Fritz shrugged: OK, one more shot - but that's it.

They upped the dose of explosives and blasted open a gaping hole that revealed promising glints of a mineral vein. Fritz thrust his arm into the hole and began rummaging around. He fished out bits of quartz, feldspar and apatite - a phosphate mineral like that is found in bones and teeth. He probed deeper, until finally his hand closed around something big, dense, faceted and thrilling.

Without even looking, Fritz knew he'd struck green. The prospectors had unearthed what would come to be called the Patricia Emerald: a dazzling 12-sided crystal with a weight of 632 carats and a verdant colour so pure and vivid you'd swear the stone was photosynthesising. Fritz sold the find for tens of thousands of dollars, while Justo, predictably enough, "was given $10 and a mule," said Terri
Ottaway, museum curator at the Gemological Institute of America.

A geochemical 'recipe'

Yet, the public arguably got the best deal of all: the stone was later donated to the American Museum of Natural History in New York, USA. Today, the Patricia remains one of the largest uncut emeralds in the world. In its raw, columnar beauty, the Patricia encapsulates an often overlooked feature of gemstones, especially the ones we deem 'precious' - diamonds, rubies, sapphires and emeralds. Their real power lies in what they reveal about the dynamo that forged them: planet earth. For scientists, a gemstone is a message in a bottle. Except the message is the bottle, a glittering clue to the extreme physical, chemical and tectonic forces at work deep underground.

Moreover, many of the qualities that helped lift the Big Four to prominence in the first place - their exceptional hardness, the depth and brilliance of their colouring, their rarity - are also key to the jewels' scientific value. Precious gems are born of strife, of shotgun marriages between hostile chemical elements, and they're tough enough to survive cataclysms that obliterate everything around them. "Earth is an incredible, giant chemical laboratory, and it's a dirty place to grow crystals," said Jeffrey Post, curator of the Smithsonian's National Gem and Mineral Collection. But those impurities grant gems their colour and character "and give us vital information about the crystal structures themselves."

The rules of gem science are not cast in stone. Researchers lately have been astonished to discover that some of the world's largest and most valuable diamonds, which can sell for hundreds of millions of dollars, originated 250 miles or more below the surface, twice the depths previously estimated for earth's diamond nurseries. Some diamonds turn out to be surprisingly youthful, a billion years old rather than the average diamond's two billion to three billion years of age. Other researchers have linked ruby creation to collisions between continental landmasses and propose that the red jewels be called 'plate tectonic gemstones'.

Stealth science

A team at the University of British Columbia, Canada analysed newly discovered sapphire deposits in Canada's Nunavut territory and concluded the stones there were generated by a novel three-part geochemical 'recipe' unlike any described for sapphire formation elsewhere in the world. You start with limestone sediments containing just the right mineral impurities and you squeeze and heat the rocky mass to 800 ° Celsius. You add fluid and allow to cool. Finally, just when the growing mineral assemblage shows signs of instability, you inject another shot of fluid and lock the crystal into place. Total cooking time: about 1.75 billion years. "If one step is left out," said Philippe Belley, a geologist, "you don't get sapphires."

In the past, geologists often dismissed gemstones as baubles and gem science as oxymoronic. "Gems were considered crass commercial materials and beneath the dignity of an academic," said George Harlow, curator of earth and planetary sciences at the American Museum of Natural History.

Jeffrey calls it stealth science. "It's a great way to get people in the door," he said. "If you put up a sign that says geology, nobody comes. But if you say, 'This way to the Hope Diamond,' then everybody wants to know more." George suggested that precious gems gained their reputation in part by their association with gold.

As insoluble stones, the gems ended up concentrated at the bottom of stream beds, often in the vicinity of similarly insoluble gold. Long prized for its ductility, beauty and resistance to oxidation, gold was considered the property of rulers and kings, so why not the glittering stones found beside it? "The word diamond stems from the Greek terms for 'indestructible' and 'that which cannot be tamed'," George said. Diamonds are not indestructible, but they are the hardest substances known, given the top score of 10 on the Mohs scale of hardness - that is, resistance to scratching.

Behind a diamond's untameability is its 3D structure, a repeating crystalline lattice of carbon atoms, each one strongly bonded to four neighbours atop, below and to either side. Persuading large numbers of carbon atoms to lock limbs in all directions requires Stygian whips of high heat and pressure, as until recently could only be found underground. In theory, the earth's mantle, which is thought to hold about 90% of the planet's carbon supply, is practically glittering with diamonds at various stages of formation.

Getting those jewels to the surface in bling-worthy condition is another matter. Diamonds must be shot up from below quickly or they'll end up as so much coal in your stocking. Researchers have discovered diamonds that had blundered crustward slowly enough for their carbon bonds to expand, leaving a stone with the shape of a diamond but the consistency of graphite.

Gareth R Davies, a professor of geology at Vrije Universiteit Amsterdam, and his colleagues have recapitulated the reversion process in the laboratory. "Yes, we get diamonds and turn them to graphite for research," he said. Researchers can also fabricate diamonds in the laboratory, although the results are more often destined for industry.

Bright colours

Colouration mechanics figure more prominently still in the genesis of coloured gemstones. After all, sapphires and rubies are built of the same basic mineral, corundum, a crystallised collaboration of aluminium and oxygen that would be transparent and colourless if not for some artful chemical doping. With a Mohs hardness score just a point shy of diamond's, corundum becomes a red ruby through the timely addition of chromium atoms. Recent research suggests chromium is shoved up to the crust from earth's mantle when continental landmasses bang together.

A sapphire is a corundum crystal of any colour but red, although many people consider a true sapphire to be blue. In that case, the blue results from electrons bouncing back and forth between near-homeopathic doses of iron and titanium atoms sprinkled throughout the crystal. "It's called intervalence charge transfer," said George. "You almost can't measure the amount of iron and titanium, but the small effect produces a dramatic colour."

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