DISCOVERY OF DIAMONDS THAT CHANGE COLOR AT LIQUID NITROGEN TEMPERATURES

Among the rarest of diamonds, are chameleon diamonds the only natural diamonds that change color (Fritsch et al., 2007). For the first time since 1866, another type of natural color changing diamonds has been identified. We examined five samples that displayed a distinct temporary change of color from gray to yellow or blue when subjected to extremely low temperatures. These “cryogenic diamonds” were all type IIb with the exception of one type IIa (possible boron not detected). The mechanism of this color change effect is theorized to be from a new defect, a boron-nitrogen pair, or a cause yet to be discovered.

A total of five samples were studied during this investigation. A combination of advanced spectroscopic methods was employed in order to gain better understanding the of the growth morphology and lattice defects contained within these diamonds. Photoluminescence (PL) spectroscopy was collected using a Renishaw inVia Raman microscope at 325, 488, 514, 532, 633, and 830 nm laser excitation, at liquid nitrogen temperature (-196 °C) for all samples. Each of the these diamonds displayed trace amounts of nickel related defects at 882-884 nm in the 830 nm laser and a 776 nm peak active only to the 633 nm laser. Fourier transform infrared spectroscopy (FTIR) was collected using a Thermo scientific iS50 Nicolet FTIR spectrometer using KBR beamsplitters and a DRIFT (diffuse-reflectance infrared Fourier transform) accessory. Boron was found in all samples with exception of one sample, which was typed as type IIa with possible Boron undetected with current testing methods.

When optical centers, also known as defects, in a diamond’s crystal lattice absorb visible light they produce color in the diamond (Collins 1982). Optical centers within a diamond form from the result of deformation within the diamond lattice, impurities and vacancies. Coloration processes imply electron transitions between at least two different levels (Fritsch et al., 2007). There could be shift of energy within the electron band gap. The stable state of these diamonds are fancy gray. Placing the diamonds in liquid nitrogen thus decreasing the temperature causes a shift in energy, which could cause the change of color. The cause of this color change and environment in which these diamonds were formed are important in the understandings of diamond genesis.