The Geology of . . . Rubies

A ruby's dazzling color masks the mysterious origins of its birth

By Anne Sasso|Thursday, November 25, 2004
RELATED TAGS: EARTH SCIENCE

Sometimes George Harlow looks more like a medieval magician than the curator of gems and minerals at the American Museum of Natural History. Sweeping an ultraviolet light wand over a box of rocks in a darkened room at the museum, he ignites the stones—uncut rubies—in a burst of fiery red light that is otherworldly. “It’s like Day-Glo colors,” he says. “They are brighter than they should be. You look at them and say: ‘Wow! Look at that red! What’s going on?’ ”

That remarkable radiance—caused by any ultraviolet light, including UV rays from the sun—has given rubies a special place in history. Long before Marco Polo found his way to Asia, Burmese warriors were embedding the stones under their skin to make them invincible in battle. Sanskrit medical texts were prescribing rubies as a cure for flatulence and biliousness. And Hindu lore was preaching that a ruby’s light could not be extinguished nor hidden by clothing. Geologists can explain the glow: Ultraviolet light causes the chromium in rubies to fluoresce. But there is much about rubies that scientists cannot account for. The biggest question, the one that has geologists on both sides of the Atlantic at odds with each other, is how rubies formed in the first place. Geologists simply do not know. That rubies even exist, says Peter Heaney, geosciences professor at Penn State University, is something of a “minor geological miracle.”

Rubies are a type of corundum, a rare mineral made up of densely packed aluminum and oxygen atoms, which are normally colorless. When other atoms are substituted for a few of the aluminum ones, bright hues emerge. Small amounts of chromium impart the deep red color of ruby, traces of titanium and iron produce the stunning blue of sapphire, and chromium and ferric iron create the delicate orange shades of the extremely rare and costly padparadscha.

None of this can take place, however, if silica or large amounts of iron are present. And therein lies the mystery. Since silica is one of the most abundant elements in Earth’s crust, how is it that rubies managed to avoid it but at the same time connect with the exceedingly rare chromium? And how did rubies avoid iron, another common element? Sapphires and padparadscha need some iron, but rubies, by definition, have very little at all. “The fluorescence [of a ruby] is tied to its composition, to the low iron. That’s hard to do in geology, to get the iron that low,” says Harlow. “Corundum is rare enough as it is. So, adding all these things together, ruby is very rare.”

The majority of the world’s ruby deposits (but not necessarily the best) are in a discontinuous band of marble that stretches 1,800 miles along the southern slope of the Himalayas from Tajikistan through Afghanistan, Pakistan, Kashmir, Nepal, and on into China and Vietnam. The model of ruby formation that many geologists, including Harlow, accept involves tectonics: two continents—India and Asia—smashing together to form the Himalayas.

Around 50 million years ago, the Indian subcontinent moved toward Asia, constricting the Tethys Sea, an ancient ocean that lay between. On the floor of the Tethys were deposits of limestone, sedimentary rocks of calcium carbonate (the stuff of Tums). “It turns out that many limestones are dirty,” says Heaney. The Tethyan limestone was composed of every mineral that washed off the rocks of the surrounding land, including all the ingredients necessary for rubies: aluminum, oxygen, and chromium, plus silica.

As the Tethys closed up, its limestones were pushed deep into the earth, where they were cooked and squeezed at inferno-like temperatures (1,112 to 1,238 degrees Fahrenheit) and pressures (3 to 6 kilobars). The result? They metamorphosed into sparkly marble—the kind Michelangelo loved to work with. At the same time, molten granite intruded into the marble, releasing fluids that percolated up through the rock. That process, called metasomatism, removed the silica but left the alumina behind. For the next 40 million to 45 million years, the two continents slowly squeezed together, raising the Himalayas. Erosion eventually exposed a necklace of ruby deposits along the scar where the two plates collided.

Studies done in France by Gaston Giuliani of the Institute of Research and Development, along with Virginie Garnier and Daniel Ohnenstetter of the Petrographical and Geochemical Research Center, back up the traditional view—in part. They link the timing of ruby formation to the rise of the Himalayas. “When we dated the ruby deposits, we noticed that they were directly related to the continental collision and to the Himalayan orogeny [mountain building],” says Giuliani. “So the ruby is in effect an ideal marker of this continental collision.”

But the French team also noticed that while the Himalayas’ ruby-hosting marbles extend over large areas, the rubies themselves occur only erratically in patches. “The occurrence of ruby is very isolated and localized. We don’t find rubies everywhere that there is marble. So then we had to ask, why do we only find ruby in certain locations? Because if it’s a metamorphic phenomenon, normally it affects the whole ensemble of marble,” says Giuliani. “But that wasn’t the case, so there’s a big secret here.”

The secret, the French believe, is salt. Not only were the limestones dirty, they were salty as well. The Tethys, they say, was so shallow in places that it would occasionally dry out, leaving behind a thin rind of salt from evaporated seawater. The salt mixed with detritus washing off the land to form the unique limestone that gave birth to rubies. Once heated, the salt acted like a flux, assuring that the aluminum became mobile enough to mix with the chromium.

Further clues to salt’s role lie deep within the microscopic world of the ruby crystal. There Garnier found tiny drops of fluid, immortalized snapshots of the liquids swirling within the marble when the rubies crystallized. Minuscule crystals of sodium chloride and anhydrite (found in sea salt) float within the liquid. But what of ruby’s enemy, silica? Garnier claims that there wasn’t enough present in the original rocks to do much damage. And what of the role of granite? Giuliani says it had no role at all.

Harlow disagrees. “The fundamental issue is, if you metamorphose a marble, the silica content is much greater than that of aluminum, and you’re never going to make corundum—although we all know that there are marble-hosted corundum deposits. So how do you do it?” he asks. “Simple. You need a fluid. You need some transport mechanism to reduce the silica in the rock.” Intrusions, like granites, offer a convenient source of fluids. “It’s a simple mechanism, even though it’s not yet proven for rubies,” he adds.

No model of ruby formation will be considered definitive until geologists can explain the legendary stones of the famed Mogok mine in Myanmar (formerly Burma), source of some of the world’s finest rubies and spinels. While Mogok gems are indeed hosted in marble, they often grow alongside beautiful topaz and moonstone, minerals that are igneous (crystallized from rising magmas) rather than metamorphic in origin. The huge size of these crystals implies a type of magma called pegmatite, a juicy water-rich melt that provides unusual conditions, allowing minerals to grow to enormous sizes. This suggests that different processes than those hypothesized for the creation of other rubies were at work. “The minerals blew my mind,” Harlow says. “I started seeing things that really challenged the concept that rubies are metamorphic.”

Sadly, it may be some time before geologists can sort this out. Politics in Myanmar have long blocked scientists —especially Western scientists—from       entering the country to take a look. “The fundamental problem with Myanmar is that you can’t get in there to do anything,” Harlow says. “And the people who have done the geology are Myanmar geologists who, unfortunately, are suffering with being 40 years behind in science.”

If Western scientists were allowed complete access to Mogok, would they find an answer to the question of how rubies formed? Harlow isn’t sure. “Yes, Mogok’s special,” he says. “But is it going to defy the other models or defy other interpretations? I don’t think fundamentally it will. There’s a

Tom Chatham of Chatham Created Gems in San Francisco knows how rubies form—sort of. He can cook up a batch of synthetic rubies in a year. “There are still a lot of mysteries in our process,” he says. “It’s not cut and dried; we get a lot of worthless stones.” Even so, Chatham’s lab-created stones can fool everyone but gemologists.
strong similarity among a lot of the deposits, even though the details tend to be different. I think we’re still a ways away from answering these questions.”

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