Taking a close look at unique diamonds

The song says "diamonds are a girl's best friend", but scientists at the Naval Research Laboratory (NRL) are finding that diamonds are a researcher's best friend too. The NRL, which has been involved in pioneering work involving chemical vapour deposition of diamonds and the use of diamond materials in advanced technologies relevant to the Department of Defence since 1987, has undertaken some new projects in diamond research.

 

In collaboration with the Smithsonian Institution Museum of Natural History, NRL researchers have begun studying unique and historic natural-coloured diamonds to understand and characterise the defects/impurities which cause the colour. Many of the properties of a diamond necessary for technology are impacted by defects and impurities present in the lattice. NRL has been complementing its studies of these defects and impurities in chemical vapour deposition diamond materials with its studies of natural diamonds at the Smithsonian.

 

Since late 2005, a team of NRL researchers led by Dr James Butler of the Chemistry Division has been examining unusual natural-coloured diamonds available to the Smithsonian. These included many of the diamonds in the Smithsonian Collection, such at the ‘Hope’ and the ‘Blue Heart’, as well as a collection of 240 fancy-coloured diamonds in the Aurora Butterfly collection on loan to the Smithsonian.

 

This year, NRL has been working with the Smithsonian and the Gemological Institute of America to study another famous blue diamond, the ‘Wittelsbach-Graff’ diamond. Both the Hope and the Wittelsbach-Graff diamonds are believed to have originated from the same region in India in the 17th century; they have a similar blue colour and nearly identical red/orange phosphorescence when excited by ultraviolet light. Hence, it has been speculated that they might have originated from the same stone.

 

The Wittelsbach-Graff diamond was last seen in public in 1958; then in 2008, Laurence Graff, a diamond dealer, bought it at auction for 16.4 million GBP. Graff had the stone cut and repolished, reducing it from a 35.5-carat stone to a 31-carat stone, compared to the Hope diamond, which is 45.52 carats.

 

The research team studying the Wittelsbach-Graff diamond used a variety of spectroscopic and microscopic analyses to determine the extreme similarity of the gems, but also observed distinct differences in the dislocation and strain microstructure, which suggests that the gems probably did not originate from the same rough stone.

 

The Wittelsbach-Graff is on display at the Museum of Natural History from February to August 2010 along with the Hope diamond. This work continues the ongoing collaboration between NRL scientists and the Museum of Natural History on the Hope diamond and other blue diamonds at the Smithsonian which has examined the phosphorescence (due to donor-acceptor recombination), the boron concentration using secondary ion mass spectroscopy, and soon to be published work on the spectroscopic and structural properties of a collection of pink diamonds.

 

Another aspect of NRL's diamond research collaboration with the Smithsonian involves an interdisciplinary effort to study rare pink diamonds. Many natural pink diamonds derive their colour from coloured bands or lamellae in an otherwise colourless diamond.

 

Led by Jeff Post, Eloïse Gaillou, and Tim Rose of the Smithsonian Museum of Natural History; NRL researchers James Butler (Chemistry Division), Rhonda Stroud and Nabil Bassim (Materials Science and Technology Division); along with Alexander Zaitsev, CUNY; and Marc Fries, JPL/Cal Tech studied a suite of natural pink diamonds. The research team used a variety of spectroscopic and microanalytical tools to study the structure, defects and impurities in and around the coloured lamellae.

 

Pink diamonds are extremely rare, on a par with blue diamonds in rarity and value. But unlike most blue diamonds where the colour is caused by an impurity atom - boron, pink diamonds seem to derive their colour from structural or a combination of structural and impurity related defects.

 

While the research team has not identified the exact structure of the defects causing the pink colour, they have determined that it is contained in narrow-coloured lamellae in an otherwise clear matrix of diamond. Using a focused ion beam microscope, NRL researchers extracted cross-sections of the pink lamella for detailed examination in a transmission electron microscope (TEM). TEM examination of the lattice structure, combined with spectroscopic analysis, suggests that the lamellae are the result of plastic deformation, which occurred while the diamond was still in the earth's mantle and before it was transported to the surface in ancient volcanic eruptions.

 

They will continue their studies to characterise a suite of rare pink diamonds to see if they can fully identify the nature and cause of the defects which cause the pink colour. "The pink lamellas are twin domains, with atoms arranged to mirror almost exactly those of the surrounding clear diamond. The real question is what subtle shift in the atomic arrangement makes the twins pink but leaves the nearly identical sibling colourless? The sub-angstrom imaging capabilities of the latest generation of electron microscopes should tell us the answer," says Stroud.

 

"Understanding these unique-coloured natural diamonds provides knowledge useful to both technologists and gemologists," Butler explains. "A better understanding of these defects and impurities (dopants) allows us to tailor the materials properties of diamond materials: from electrically insulating to semiconducting; from optically transparent to a variety of colours; or to provide the isolated quantum states for quantum cryptography or quantum computing."