Natural Orange Diamond with Unusual Absorption Features
Orange is a very desirable color for diamond in the gem trade. GIA’s New York laboratory received two type Ia (nitrogen-bearing) orange diamonds for colored diamond grading service, one 0.16 ct Fancy Vivid orange and the other 0.23 ct Fancy Intense orange (figure 1). Both diamonds showed orange fluorescence to long-wave and short-wave UV, along with few inclusions (clarity grades of VS2 and VVS1, respectively) and strong color zoning related to crystal growth (figure 2).
This diamond hue is often caused by a broad-band absorption at 480 nm. While the 480 nm band was observed in both diamonds, it was supplemented by the presence of a 525 nm peak and the 550 nm band along with a series of absorption features from ~700 to 780 nm with pronounced peaks at 735, 753, and 769 nm (figure 3). The 830 nm photoluminescence (PL) spectra showed very strong nickel peaks at 883/884 nm, in addition to numerous other PL features (A.M. Zaitsev, Optical Properties of Diamond: A Data Handbook, Springer-Verlag, Berlin, 2001). Thus, the 480 nm band was not the exclusive cause of color. The 550 nm band is the most common cause of color in natural pink diamond but, when present with other color centers, can contribute to many different bodycolors.
The optical defects in these orange diamonds differed from those in diamonds exclusively colored by the 480 nm band (M.Y. Lai et al., “Spectroscopic characterization of diamonds colored by the 480 nm absorption band,” Diamond and Related Materials, Vol. 142, 2024, p. 110825). The combination of an unusual absorption band in the near infrared with the 480 and 550 nm absorption bands creates a transmission window that produces the rich, warm orange hue, devoid of any secondary modifying color. This appears to be a newly identified cause of orange color in diamonds.
