Recently, large colorless and near-colorless HPHT-grown diamonds by the Russian company NDT have been investigated, with sizes up to 5.11 ct (U. D’Haenens-Johansson et al., 2015, “Large colorless HPHT-grown synthetic gem diamonds from New Diamond Technology, Russia,” Fall 2015 G&G, pp. 260–279; Spring 2015 G&G Lab Notes, pp. 65–66). The largest faceted colorless HPHT-grown synthetic diamond reported to date is a 10.02 ct E-color, VS1-clarity specimen, reported by IGI Hong Kong in 2015. In January 2016, GIA’s New York laboratory examined a 5.03 ct fancy-color HPHT-grown type IIb synthetic diamond (figure 1, left) produced by NDT, the largest faceted blue laboratory-grown diamond studied so far.
This emerald-cut synthetic diamond was color graded as Fancy Deep blue. This is a very attractive color with no other color component, a prized rarity among natural type IIb diamonds (the Blue Moon, for instance, was graded as Fancy Vivid blue). When viewed under a microscope, faint but sharp color zoning could be seen (figure 1, center), indicative of the uneven impurity incorporation of HPHT synthetic diamonds. No strain was observed under crossed polarizers, indicating a very low dislocation density, which is also characteristic of HPHT-grown diamonds. It had VS1 clarity, with only very small metallic inclusions and a cavity observed at the girdle (figure 1, right). Fluorescence and phosphorescence images collected using a DiamondView instrument revealed the sample’s cuboctahedral growth pattern (figure 2, left), another feature of HPHT synthetics. The long-lasting chalky blue phosphorescence was further analyzed using spectroscopy, and the emission was found to originate from two broad bands centered at approximately 500 and 575 nm (figure 2, right). These bands have previously been reported in NDT’s type IIa and IIb HPHT synthetic diamonds (D’Haenens-Johansson et al., 2015).
Absorption spectroscopy for the mid-infrared region confirmed the sample was type IIb, with strong boron-related features at 1290, 2458, and 2800 cm–1. The average bulk boron concentration was 0.82 to 1.12 ppm, calculated according to the equation NA-ND = (1.00 ± 0.15) × H1290 ppm cm–3 , where NA is acceptor concentration, ND is donor concentration, and H1290 is peak height at 1290 cm–1 (A.T. Collins, “Determination of the boron concentration in diamond using optical spectroscopy,” Proceedings of the 61st Diamond Conference, Warwick, UK, 2010). Otherwise, this large synthetic diamond exhibited an extremely low concentration of optical defects. Photoluminescence spectroscopy was conducted at liquid nitrogen temperatures using a range of laser excitations covering the UV-visible-IR range. The PL spectra only revealed emission from a single defect species, a Ni-related emission multiplet with peaks at 483.6/483.8/484.1/484.4 nm (484 nm center) detected using 324.8 nm laser excitation (A.T. Collins, “The characterisation of point defects in diamond by luminescence spectroscopy,” Diamond and Related Materials, Vol. 1, 1992, pp. 457–469). As with previous type IIb synthetic diamonds, its visible-NIR spectrum showed a transmission window in the blue region and an absorption in the red, caused by the presence of boron, resulting in the observed blue bodycolor.
This 5.03 ct sample is the largest HPHT-grown blue synthetic diamond examined at a GIA laboratory. As the size and quality of synthetic diamonds improve, careful identification is essential. Representative HPHT synthetic diamond characteristics seen in this specimen, such as the lack of tatami strain patterns (which are typically observed in natural type IIb diamonds), faint but sharp color zoning, and small metallic inclusions from the metal-catalyst flux, can be detected using a gemological microscope, emphasizing its continued importance in gem identification. Examination of this large IIb synthetic diamond, combined with those previously reported from NDT, illustrates the rapid progress in HPHT growth technologies. This is a development that will eventually impact the jewelry industry.