CL IMAGING STUDY OF A MIXED TYPE DIAMOND

Mixed type IIa (no detectable nitrogen impurity) + IIb (Boron containing) diamonds are commonly found among HPHT lab-grown synthetics, but are rarely found in nature. We had the opportunity to investigate one such rare diamond, a IIa + IIb mixed type brownish yellow stone which was determined to be a natural diamond.

We have undertaken a thorough examination of the growth zones present in this diamond in order to better understand the conditions of the surrounding environment during its formation. Using a combination of fluorescence imaging, FTIR spectroscopy, photoluminescence (PL) hyperspectral imaging, and cathodoluminescence (CL) imaging, we were able to clearly define the IIa and IIb sections of the growth zones present in this diamond.

Initially, we found that collecting an FTIR spectrum from different points on the diamond showed different spectra corresponding to a type IIa and a type IIb diamond, indicating that the diamond was potentially a mixed type. Furthermore, upon obtaining fluorescence images using a DiamondView instrument, we saw that there were two clear sectors present in the stone, with one sector showing strong phosphorescence. We hypothesized that the phosphorescent sector was the type IIb sector and the dark sector was type IIa. After performing PL hyperspectral imaging in the same area imaged by the DiamondView, we found that optical defects commonly found in type IIb diamonds were present in the sector initially hypothesized to be IIb. CL imaging using a scanning electron microscope revealed a clear light and dark growth boundary between the IIa and IIb sectors. The IIb sector showed a dark characteristic mosaic-like dislocation network common to type II diamonds, while the IIa sector appeared lighter.

This study offers insight into the environment of diamond crystal formation. Coexistence of type IIa and IIb in a single crystal diamond revealed the drastic variations of diamond growth environments. At GIA we have used CL imaging to separate natural from lab grown diamonds, which has proven to be a powerful technique for examining growth structures and growth zones in diamonds.