DEBBIE ELLIOTT, HOST:
There's a diamond going up for sale. It's said to be out of this world. Get this; it's a black 555.55-carat diamond that fits in the palm of your hand.
A MARTÍNEZ, HOST:
The auction house Sotheby's says this rare gem may be a treasure from interstellar space. Now, that caught the attention of Tim McCoy.
TIM MCCOY: Not so sure it came from outer space.
ELLIOTT: McCoy's a curator of the meteorite collection at the Smithsonian National Museum of Natural History in Washington, D.C.
MCCOY: Just at the Smithsonian alone, we have about 30,000 different meteorites. None of them contain anything like this. It's millions of little crystals, all, like, a few microns in size, all stuck together to make this big 555-carat - what is really a rock more than a single crystal.
MARTÍNEZ: McCoy says the mysterious diamond is known as a carbonado, found only in Brazil and the Central African Republic.
ELLIOTT: No one really knows how carbonadoes formed. Mccoy says it could have happened underground 4 billion years ago.
MCCOY: We think of outer space as this really exotic place; but think of inner Earth as a really exotic place. We don't really know very well what the deep, deep Earth is like. You know, every rock has a story, and we just have to know how to listen to it. And I think this is a rock that we can't quite understand its story yet. But it's going to be a really good one when someone figures it out.
MARTÍNEZ: Sotheby's gave the diamond a nickname - the enigma. It's expected to sell for 6.8 million bucks at auction next month.
(SOUNDBITE OF RIHANNA'S "DIAMONDS (INSTRUMENTAL)") Transcript provided by NPR, Copyright NPR.