ANALYSIS OF 480 NM BAND IN A BI-COLORED DIAMOND

The broad absorption band with peak maximum around 480 nm (namely ‘480 nm band’) is responsible for yellow–orange colors in natural diamonds. However, it is one of the poorly understood defects. In this research, we analyzed 480 nm band which is detected in a 4.06 ct, Novelty step-cut diamond. It is a single diamond crystal with distinct color zones, mainly brown-orange and light gray. The brown-orange zone, which is composed of dark and light zones, contains small clusters of plate-like inclusions. Eclogitic inclusions, such as omphacite and pyrope crystals can be located throughout the diamond.

Photoluminescence (PL) and Fourier-Transform Infrared (FTIR) spectral analyses reveal differences in concentration and presence/absence of defects between colored zones. Both zones contain nitrogen aggregates (A>B; A = a pair of nitrogen; B = a group of four nitrogen) but light gray zone has higher concentration compared to brown-orange zone. However, the brown-orange zone is rich in both hydrogen (sharp peaks in FTIR spectral range from 3050 to 3300 cm^-1) and nickel defects (PL peaks at 882-884 nm; Ni_s⁰). Interestingly, the 480 nm band is detected only in the brown-orange zone. This band was previously claimed to be related to oxygen atoms (Hainschwang et al., 2008). No solid CO₂ in FTIR spectra was observed in these colored zones.

This bi-colored diamond was formed by multiple growth episodes. It contains intricate growth structure which can be observed in the DiamondView fluorescence images. However, we do not see relationship between color zoning and diamond growth structure. All these observations suggested that defects related to 480 nm band, which is directly related to the brown-orange zones, were formed after host diamond growth. We propose that these defects could be post-growth related defects, e.g. vacancy clusters that are formed by plastic deformation, in addition to other possibilities. Localized formation of vacancy clusters are causing brown-orange color zoning.