Problem: The colour of natural pink Argyle diamonds is due to an unidentified crystalline defect that causes the colour of the diamonds to be affected by certain wavelengths of light. Rio Tinto diamonds wanted to characterise this change, find the optimal wavelength at which to perform a reversal and to discover the underlying cause of the defect and its behaviour.
Solution: Confocal microscopy at the University of Western Australia allowed direct imaging of optically active defects within single-crystal diamonds. This was used to show the structure and orientation of graining to which the pink colour is restricted. It was found that the graining lies along the planes of easy slip within a crystal. The colour-causing defects are being mapped in 3-D to make out the boundaries where pink regions and clear regions meet. This provides guidance to help identify areas for more detailed investigations. Identifying the characteristics of this defect will provide Rio Tinto with methods for verifying authenticity of Argyle pink diamonds. The properties of this crystalline defect could also be of use in research into optical systems and technologies.