Rare Phosphorescence Seen In CVD Lab-Grown Diamond: GSI

Lab-grown diamonds have gained increased popularity in recent times. Today’s cutting-edge technology has made it possible to replicate a diamond’s natural growth in highly controlled environments. The result has been the production of lab-grown diamond in a few days, or weeks, with minimum investment. Lab-grown diamonds are identical to a mined diamonds chemically, physically, and optically, but cost about 50% less.

Chemical Vapor Deposition (CVD) and High Pressure High Temperature (HPHT) are the most common lab-grown diamond manufacturing processes, and each of the methods have their own distinguishing factors, enabling gemmologists and well-equipped labs to identify them.

CVD lab-grown diamonds shows distinct fluorescence when exposed to a conventional SWUV (Short-wave Ultra-Violet) lamp (280-315 nm). Typical SWUV reaction observed in CVD lab-grown diamonds are orange, yellow, green, violet or blue fluorescence colours. Prolonged exposure to SWUV may have some effect, and a change in fluorescence colour can be observed (Eaton-Magaña & Shigley 2016, Wang et al., 2003, 2005, 2007, 2010, 2012).

Recently our interest was drawn to a CVD lab-grown diamond weighing 1.547 carats, emerald cut, VS1 clarity and H colour, which was submitted to the Gemological Science International (GSI) Mumbai Lab for Post Growth Treatment Identification.

The absorption spectra in the mid-infrared region showed typical absorption features of type IIa, as seen in CVD lab-grown diamonds. No additional absorption features were observed. (Fig.1)

Fig.1: The mid-infrared spectrum indicating type IIa of CVD 1.547ct, emerald cut.

Photoluminescence spectroscopy with 532 nm excitation showed peaks at 737, 637 and 575 nm (Fig.2). The 737 nm and 766 nm emission systems are associated with silicon-vacancy centres, including moderately strong NV emission lines at 575 and 637 nm (Clark et al.,1995).

Fig.2: Photoluminescence spectroscopy with N-V centres and Si-V peaks of CVD 1.547ct, emerald cut.

Raman microscope with 532 nm spectrometers showed extremely strong NV emission lines at 575 and 637 nm, absence of 596/597 nm doublet indicates post-growth treatment. (Fig.3) (Clark et al.,1995).

Fig.3: Raman microscope spectroscopy with N-V centres related peaks of CVD 1.547ct, emerald cut.

DiamondView revealed a strong greenish yellow fluorescence when observed length wise, and interestingly, on a rotation of 180 degrees, a strong orange fluorescence was observed. (Fig.4)

Fig.4. Greenish yellow fluorescence (left), on rotation of 180 degrees strong orange colour observed.

Most CVD in DiamondView have shown orange, red, blue and sometimes mottled distribution of purple, red, and blue fluorescence colour. The phosphorescence is usually inert, but in some rare instances, variable degrees of weak blue-green phosphorescence has been observed. Studies have revealed that these variable fluorescence colour and phosphorescence intensities could be related to the growth of CVD lab-grown diamonds. The change in the fluorescence colours could be due to the defects in the internal crystal plane. (Lu, Q. et al.,2021).

Due to prolonged observation in DiamondView, when the DiamondView was turned off, an intense red phosphorescence for a few milliseconds (10-15 milliseconds) was observed which quickly changed to yellow (Fig.5). Such observations have not been reported earlier in CVD diamonds. This intense red phosphorescence was observed only after prolonged exposure to DiamondView.

Fig.5. Phosphorescence seen post UV excitation for milli seconds only.

The possible reason for different fluorescence colours, in different direction and phosphorescence, is assumed could be due to inconsistent lattice defects during the CVD growth process.


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