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Black diamonds come from space, scientists claim

Jan. 21, 2007
Courtesy National Science Foundation
and World Science staff

If in­deed “a dia­mond is forever,” the most prim­i­tive ori­gins of Earth’s so-called black dia­monds were in deep, uni­ver­sal time, ge­ol­o­gists have found—they came from space.

A pol­ished and cut black dia­mond (in­set), along with a rough­ly 12 mm wide un­pro­cessed one. (Cour­te­sy NSF)

The study of the rare black gems, al­so called car­bo­na­do dia­monds, comes from a re­search group in­c­lud­ing Joz­sef Garai and Ste­phen Hag­gerty of Flor­i­da In­ter­na­tion­al Uni­ver­si­ty.

The dusky rocks, Hag­gerty said, may have once been as­ter­oid-sized—a kil­o­me­ter (0.6 miles) or more in di­am­e­ter when they first reached Earth.

The sci­en­tists based the claims on various fac­tors. They an­a­lyzed the dia­monds’ con­tents and found them to be si­m­i­lar to cer­tain oth­er dia­monds found in me­te­orites, which al­so or­i­ginate in space. 

Minute amounts of hy­dro­gen in the car­bo­na­do rocks also be­trayed a sim­i­larity to dia­mond dust found in hy­dro­gen-rich en­vi­ron­ments among stars, his group said. In previous re­search, Hag­gerty sug­gested car­bo­na­do dia­monds formed in stel­lar ex­plo­sions called su­per­novae.

The term car­bo­na­do, coined by the Por­tu­guese in Bra­zil in the mid-1700s, al­ludes to the dia­monds’ re­sem­b­lance to po­rous char­coal. The stones are found on­ly in Bra­zil and the Cen­tral Af­ri­can Re­pub­lic.

Con­ven­tion­al dia­monds are mined from ex­plo­sive vol­can­ic rocks called kim­ber­lites that shoot from pro­found depths to the Earth’s sur­face, said So­nia Es­per­anca of the Na­tion­al Sci­ence Foun­da­tion’s Di­vi­sion of Earth Sci­ences, which funded the re­search. “This pro­cess pre­serves the unique crys­tal struc­ture that makes dia­monds the hard­est nat­u­ral ma­te­ri­al known.”

Diamonds consist of a dense, crys­tal­line form of car­bon. But from Aus­tral­ia to Si­be­ria, from Chi­na to In­dia, con­ven­tion­al dia­monds come from vir­tu­al­ly iden­ti­cal ge­o­log­i­cal set­tings, said Hag­gerty—none of them com­pat­i­ble with black dia­mond for­ma­tion.

About 600 tons of con­ven­tion­al dia­monds have been mined, traded, pol­ished and adorned since 1900. But not one black dia­mond has been dis­cov­ered in min­ing fields, added Hag­gerty, whose study appeared on­line Dec. 20 in the jour­nal As­t­ro­phys­i­cal Jour­nal Let­ters. 

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If indeed “a diamond is forever,” the most primitive origins of Earth’s so-called black diamonds were in deep, universal time, geologists have found—they came from space. In a paper published online on Dec. 20, 2006, in the journal Astro physical Journal Letters, Jozsef Garai and Stephen Haggerty of Florida Inter national University, along with other researchers, claim an extraterrestrial origin for the unique black diamonds, also called carbonado diamonds. The scientists based the claim on a variety of factors. They analyzed the diamonds’ contents and found them to be similar to certain other types of diamonds found in meteorites, which also originate in space. The presence of hydrogen in the carbonado diamonds indicates a similarity to diamond dust found in hydrogen-rich environments among stars, he and colleagues said. The term carbonado was coined by the Portuguese in Brazil in the mid-18th century; it’s derived from its visual similarity to porous charcoal. Black diamonds are found only in Brazil and the Central African Republic. Conventional diamonds are mined from explosive volcanic rocks called kimberlites that quickly carry them from profound depths to the Earth’s surface, said Sonia Esperanca, program director in the National Science Foundation’s Division of Earth Sciences, which funded the research. “This process preserves the unique crystal structure that makes diamonds the hardest natural material known.” From Australia to Siberia, from China to India, the geological settings of conventional diamonds are virtually identical, said Haggerty. None of them are compatible with the formation of black diamonds. About 600 tons of conventional diamonds have been mined, traded, polished and adorned since 1900. But not one black diamond has been discovered in mining fields, Haggerty said. The new data would support earlier research by Haggerty suggesting carbonado diamonds formed in stellar explosions called supernovae. Black diamonds, he added, were in fact once the size of asteroids—a kilometer (0.6 miles) or more in diameter when they first hit Earth.