Golconda Diamonds From Old Mines: Windows of Earth's Mantle Below the Deccan

During the last seven years, we collected 59 flat rose-cut diamonds previously used in Mogul jewelry worn by kings and noblemen in India. The cutting style indicates that they were made at least one or two centuries ago, and the diamonds were most likely produced in areas near River Krishna on the Deccan Plateau where Golconda was the diamond trading center. Some of the diamonds are up to 1 cm in size, and several flat plates appear to be serial sections of a very large stone, because they yielded identical Fourier-Transform infrared (FTIR) spectra. These samples gave us an opportunity to study very large diamond crystals at a relatively affordable cost. We present here our prliminary results. An FTIR study showed that almost all the samples are Type Ia diamonds whose nitrogen aggregation state is A, with a minor component of the B aggregate. The spectra of most diamonds show a sharp hydrogen peak at wavenumber 3107, and several small peaks just below wavenumber 3000, due to the presence of hydrocarbon molecules. Mineral inclusions are eclogitic, represented by orange garnets and pale green omphacitic pyroxene. Only one diamond contains 11 crystals of olivine. Similarities among the diamonds are surprising, because they were acquired from different vendors at different times. We speculate that Earth's mantle in this region might be uniform chemically, or, the diamonds came from a single productive mine.