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  Jadeite and corundum provide clues to earth's earlier history


June 22, 2013


For years geologists were puzzled by jadeite. They didn't make any sense, said George Harlow, a geologist at the American Museum of Natural History, speaking to the New York Times. He is a member of a research team that has been studying gemstones to learn more about the earth's history.

Today it is believed that jade provides physical evidence of oceans buried underground. As ocean tectonic plates were pushed beneath continental tectonic plates, the extreme pressures that were created forced atoms into new molecular arrangements. One of these was the mixture of sodium, aluminum, silicon and oxygen that is better known as jadeite.

Writing in the journal Earth and Planetary Science Letters, Dr. Harlow and his fellow researchers reported finding jade-bearing deposits at the bottom of the Caribbean Sea that are 30 million years older than the surrounding rock. This, they suggest, is what you would expect to see if jade-producing fluid rises up from sinking oceanic crust before it is joined by other materials from a sinking ocean plate.

Sapphires, too, are geological story-tellers, and especially corundum from the Indian subcontinent. There, mountain ranges like the Himalayas were formed when two continental plates collided 50 million years ago.

As the Indian tectonic plate slid underneath the Asian tectonic plate, rocks was squeezed and heated, including shale, which was formed from sediments that washed off land. When crushed, the shale produced crystals of aluminum and oxygen, which eventually became sapphires.

Some of those crystals, as they were pushed up toward the surface of the Earth, moved through rocks that are rich in chromium. The chromium atoms replaced the aluminum atoms, transforming the stones into red rubies.

The geologists credit the plate tectonic activity, which took place about 500 million years ago, for answering the question why there aren't any jade stones or rubies from the first nearly four billion years of the planet's existence. Earth is more than 4.5 billion years old, give or take a decade or so.

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