Ruby Land: The Gems and Geology of Myanmar's Mogok Stone Tract

by AMNH on

From the Field posts

Mogok, an historic city in northern Myanmar (Burma), lies in a valley 50 miles west of the snaking Irrawaddy River, about 3,500 feet above sea level. The shrub- and flower-covered hills rising above are dotted with small towns, villages, and garden plots, and adorned with well-tended Buddhist shrines. The spires of these gold-leaf-covered pagodas reach skyward, like gilded sculptures arising from rock-outcroppings along not just the area’s one major highway but also its dirt roads and walking paths.

Myanmar Monastery
Buddhist Myanmar is dotted with shrines and gold-leaf-covered pagodas, like these at Kyauk-Pyat-That monastery, rising from the rocks.
Image Credit: J.Newman 

Mogok is best known for its gemstones, including ruby, sapphire, spinel, peridot, and moonstone. For centuries, the Mogok Stone Tract’s hills were legendary for such amazing abundance that locals were said to come upon gems just glinting in the grass in their gardens. The area is still world-famous for gems: A sign along the highway reads “Welcome to Ruby Land,” as about 1,000 working mines and diggings are found there today; most of the world’s finest gem rubies come from Myanmar, most of these from Mogok.

Myanmar Gem Processing
These women use bamboo baskets to concentrate gems, like gold miners panning for gold, from the outwash of a gem processing plant.
Image Credit: J. Newman

“Geologically, Mogok is an unusual place,” says Curator George Harlow, who specializes in minerals and gems. Dr. Harlow has visited the country’s mineral-rich regions three times since his first trip in 1997-8—a trip that the not-prone-to-hyperbole curator described as “a jaw-dropping experience. I don’t know anyother place on the entire planet that has such a diverse suite of minerals.”

Harlow is one of the lucky few to have traveled to Myanmar over the last few decades, however. Until 2011, the country was ruled by a military junta, and travel was greatly restricted, even for researchers. Since a government transition, a series of political reforms in this Buddhist nation of about 56 million people is gradually opening its borders to scientists, businesspeople, and even more so to tourists, in some places.

In November 2013, a group of Museum geologists finally got a long-awaited opportunity: to travel to Mogok to study the complex geological evolution of “Ruby Land.” Why was it that the region was so rich in gem-quality minerals, which are, by definition, rare? Harlow was joined by Curator James Webster, who studies magma processes, and Senior Scientific Assistant Jamie Newman, on a Constantine S. Niarchos expedition supported by the Stavros Niarchos Foundation.

Harlow and Webster Myanmar
During the trip, mineralogist George Harlow and geologist James Webster (right) collected more than 200 pounds of specimens, making notes at every collecting stop.
Image Credit: J.Newman

Unlike other mineral resources, gemstones do not generally form in large ore deposits. Instead, the deposits are usually small and found only in certain geologic environments. The Mogok Stone Tract is unique because it contains several very different environments, offering one clue as to why the region is so gem-rich.

These sources include igneous (formed from magma) intrusions called pegmatites that can form large gem pockets inside other rocks. Magmas reacted with preexisting rock (which researchers call country rock) to form sapphire, moonstone, and certain rare gemstones. Metamorphism by heat, pressure, and passing fluid transformed limestone to marble and created Mogok rubies and spinels, a related red gem, during mountain-building as long ago as 200 million years. Weathering of all these rocks created river and cavern concentrations of gems, historically the richest deposits of all.

Sapphires in the Morgan Memorial Hall of Gems 

Another explanation for the presence of certain gems in Mogok, says Dr. Webster, could be the ancient circulation of extremely hot watery fluids through Earth’s crust, which might have helped minerals dissolve and re-form in veins or at contacts between different types of rock. “It’s really about hot water,” says Webster. “At one time, it must have dissolved certain things out of the rock—changing minerals to other minerals.” One hypothesis is that a portion of the Mogok deposits of the mineral corundum—a very hard mineral, second only to diamond, known to us in its red form as ruby and in many other colors as sapphire—formed in this way about 15 to 25 million years ago.

Read more about the expedition here. 

This story is adapted from an article in the Summer 2014 issue of Rotunda, the Member magazine.