STUDY OF DIAMOND GROWTH BASED ON NICKEL DISTRIBUTION

Large amounts of gem-quality colorless diamonds have been produced using High Pressure High Temperature (HPHT) synthetic growth technology. The large crystals grown usually show typical cubo-octahedron morphology with well-developed {111}, {110} and {100} sectors. In contrast, the small melee crystals produced (~2 mm in size) usually show elongated shapes with many sharp growth lines at crystal surfaces. The growth habit of these small crystals is little understood. Very strong blue phosphorescence of these crystals prohibits observation of growth habits from fluorescence patterns. Nickel (Ni) is a common impurity in the diamond crystal lattice for both natural and HPHT grown synthetic diamonds. In this study, we analyzed the distribution of trace Ni impurity in HPHT grown synthetic and natural diamonds using laser Raman mapping technology (Johnson et al., 2015). Trace amounts of Ni can be easily excited using 830 nm laser excitation (photoluminescence) at liquid nitrogen temperature with a high sensitivity.

For the large cubo-octahedron synthetic diamond crystals it was found that Ni concentration is highly concentrated in {111} growth sectors separated by {110} and {100} sectors in which the Ni is below detection limit. This type of Ni distribution forms a characteristic “cross” pattern observed in these large crystals. For natural diamonds dominated by {111} growth only we found that Ni can be detected in only some specific types of diamond. Distribution of Ni varied from stone to stone with a relatively even distribution following growth zonation and within a localized area only. Mapping of Ni distribution in these melee synthetic crystals revealed multiple stages of growth which explained the elongated crystal morphology. Details will be discussed in the presentation.