Optical defects in milky type IaB diamonds

The optical features of milky type IaB diamonds were studied systematically by non-destructive approaches including FTIR, photoluminescence (PL), and cathodoluminescence (CL) spectroscopy. From 97 type IaB diamonds ranging from 0.2 ct to similar to 100 ct submitted to GIA's New York laboratory for screening, we found that all the milky type IaB diamonds consistently displayed the hydrogen-related defect with an absorption line at 3107 cm(-1), and similar to 96% of them were accompanied by a weaker line at 3085.4 cm(-1), which is undetectable in most non-milky diamonds. Most of the diamond samples display no platelet defect or a very tiny residual platelet peak with a position at larger wavenumber in milky diamonds than in non-milky counterparts. Amber center with a weak but sharp absorption line at 4168.8 cm(-1) has been observed in similar to 73% of the milky diamonds and similar to 24% of the non-milky ones. Photoluminescence (PL) results reveal that several defects with ZPLs at 490.7, 536, 575.9 and 612.4 nm are more common in milky type IaB diamonds than non-milky ones. A zero-phonon line (ZPL) at 536 nm has been confirmed by PL mapping and CL spectra as a product of plastic deformation, and it might be linked with the H-4 center (N4V2 defect). A ZPL at 490.7 nm could be related to a nitrogen-vacancy complex. The defects present more often in milky IaB diamonds are generally associated with plastic deformation. The presence of a hydrogen-related peak at 3085.4 cm(-1) and a 536 nm center would help effectively distinguish IaB diamonds with subtle milky areas from their non-milky counterparts.