In recent years, synthetic diamond grown by chemical vapor deposition (CVD) has become an important topic in the jewelry industry. These diamonds can be synthesized under low pressure and medium high temperature (approximately 800 to 900 degrees Celsius), and have been more widely used in jewelry. Chemical vapor deposition synthetic diamonds are almost always brown or colorless type IIa diamonds and contain almost no nitrogen or boron impurity elements. As a result, a rare type Ib or IIb diamond was synthesized.
Recently, an unusual 0.94 ct fancy yellowish-brown jadeite cut was submitted to GIA's synthetic diamond grading report. It contains a group of black needle-like inclusions, appearing on a flat surface, and showing faint green transmission under optical fiber illumination. Under cross-polarized light, man-made diamonds show black and white mottled strain patterns, and their appearance is somewhat similar to the crossed "Tatami" patterns usually found in natural type IIa diamonds. Infrared spectroscopy shows that it is of type IAa, with about 10 ppm of nitrogen atom pairs aggregated in the crystal lattice. Diamond view imaging shows the red fluorescence and planar growth patterns common in CVD synthetic diamonds. Photoluminescence spectroscopy detected silicon vacancy defects introduced during the chemical vapor deposition growth process. Further study of infrared spectroscopy found a very weak peak at 3123 cm, confirming that this is a growing chemical vapor deposition synthetic diamond.
A cluster of needle-like inclusions was observed in the synthetic diamond (left, magnification 50×). The side view clearly shows that the cloud is confined to a plane (center, magnification 45×). Under cross-polarized light, speckle and cross-strain graphs can be seen (right, magnified 15×).
This marks the first time that GIA has tested CVD synthetic diamond containing accumulated nitrogen impurities. In the HPHT process, although some CVD composites are processed by HPHT to remove the brown tint, nitrogen atoms usually begin to aggregate at temperatures above 2000°C. Spectroscopy evidence shows that the sample has not undergone such treatment. . It is not clear how the nitrogen impurities enter the aggregate state. An IAA-type natural diamond may be used as a seed crystal for multi-stage CVD growth and retained in current gemstones to produce FTIR results. However, no clear evidence of this situation was found. More research is currently underway to understand this unusual CVD synthetic diamond containing accumulated nitrogen impurities.
Infrared spectroscopy shows that CVD synthetic diamond is of the IAA type, with concentrated nitrogen atom pairs. This is the first CVD synthetic diamond GIA to see nitrogen impurities of this structure. The 3123 cm-1 peak confirms that this is a well-grown CVD diamond.