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© Wikimedia CommonsSpinel the gem comes in a rainbow of colors, such as these crystals in a white marble/calcite matrix.
People don't discover a new type of moon rock every day, so consider the odds of finding one rich in a mineral that England's King Henry V wore on his battle helmet. And then imagine spotting, right on the Man in the Moon's nose, huge and previously unknown deposits of another mineral from the same family. Using data from NASA's Moon Mineralogy Mapper (M3) on India's Chandrayaan-1 spacecraft, a team of researchers recently did just that.

The new rock is a unique mixture of plain-old plagioclase -- plentiful in the Earth's crust and the moon's highlands -- and pink spinel, an especially beautiful arrangement of magnesium, aluminum and oxygen that, in its purest forms, is prized as a gemstone here on Earth. The rock was discovered on the far side of the moon by Carlé Pieters, a planetary scientist at Brown University, Providence, R.I. and the principal investigator for the M3 science team. Shortly after, massive deposits of a different type of spinel were identified on the near side by other M3 team members, led by Jessica Sunshine at the University of Maryland-College Park.

Because spinel hasn't figured strongly in discussions about the geology of the moon so far, the findings took the team by surprise. "This was definitely a 'Wow!' moment," says Noah Petro, a planetary geologist at NASA's Goddard Space Flight Center, Greenbelt, Md., who works closely with Sunshine. Additional team members come from several other institutions, including the Jet Propulsion Laboratory in Pasadena, Calif., which managed the M3 mission.

"I don't think anybody who studies the moon would have expected to see this so prominently at the surface," Petro adds.
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© Jessica Sunshine, University of MarylandIn the dark mantle deposits of the Sinus Aestuum (left), deposits of chromite spinel light up like beacons (right), but the nearby Rima Bode has no spinel. The researchers got this view by dividing the amount of reflected light at a wavelength of 2 µm by the amount at 1 µm. This ratio was calculated for every pixel in the image.

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© Jessica Sunshine, University of Maryland
The Great Impostor

Spinel the gem comes in a rainbow of colors, but the diva is the red stone, so similar to ruby in color and hardness that it earned the nickname the "great impostor." The Black Prince's Ruby -- a fiery 170-carat stone owned by Edward of Woodstock (the Black Prince) in the 14th century, worn on King Henry V's battle helmet and later incorporated into the Imperial State Crown of England -- is actually a spinel. So is the 361-carat Timur Ruby, also in the British crown jewels. Even after the two stones could be reliably distinguished, spinel was sought by royalty worldwide. The crown jewels of Iran, for example, feature a collection of these gems.