Precious Gems Bear Messages From Earth’s Molten Heart

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More recently, geologists have seen the refracted light. “My colleagues know that a gem course done as an introductory part of an undergraduate education is a really good hook,” Dr. Harlow said. “When you can show how gems form, or the properties they have, it takes a lot of chemistry and physics to understand that.”

Dr. Post 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.”

Dr. Harlow 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,” Dr. Harlow said, “and those attributed metaphysical properties made the ruler seem even more important.”

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 three-dimensional structure, a repeating crystalline lattice of carbon atoms, each one strongly bonded to four neighbors atop, below and to either side.

(In graphite, by contrast, carbon atoms are bonded together only in two-dimensional sheets, which will flake apart with the simple act of putting pencil to paper.)

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 percent 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 — say, through a volcanic eruption — 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 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. “And my wife wonders why I’m such an idiot.”

Photo

Jeweler Harry Winston holding several priceless gems in 1949, including the Sapphire of Catherine the Great (next to his thumb), the Hope Diamond (between his index and middle fingers), the Royal Spanish Emerald (at the base of his pinkie) and the Idol’s Eye (just left of the emerald).

Credit
Bernard Hoffman/The LIFE Picture Collection/Getty Images

Researchers can also fabricate diamonds in the laboratory, although the results are more often destined for industry than Tiffany. Nor can scientists create anything remotely as celestial as the Hope Diamond, the world’s largest deep-blue diamond, with a back story to match.

The diamond was discovered in India, sold to King Louis XIV of France in 1668, stolen during the French Revolution, reappeared 50 years later in the collection of the Dutch banker Henry Philip Hope — hence its name — sold by Hope’s bankrupted heir and then passed from hand to sometimes unfortunate hand, picking up an aura en route of being “cursed.”

After the jeweler Harry Winston donated the diamond to the Smithsonian Institution in 1958, blithely sending the massive jewel from New York to Washington through the mail, the diamond’s fame exploded. When Jackie Kennedy, then the First Lady, arranged a one-month loan of the diamond to the Louvre in Paris, Washington’s National Gallery of Art got Leonardo’s “Mona Lisa” in return.

Researchers have since plied the 45-carat diamond with every noninvasive tool in their arsenal, seeking to understand the precise distribution of boron atoms that lend the Hope its steely blue tint and why the diamond will glow, or phosphoresce, a spectral shade of blood orange when exposed to ultraviolet light. Dr. Post suspects the phosphorescence is the result of interactions between boron and nitrogen impurities in the diamond’s near-flawless carbon frame.

Coloration mechanics figure more prominently still in the genesis of colored gemstones. After all, sapphires and rubies are built of the same basic mineral, corundum, a crystallized collaboration of aluminum and oxygen that would be transparent and colorless 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 color 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.

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