Differing causes of colour in diamonds
A parcel of 8 African diamonds of mixed colors (Figure 1) was examined by gemological research laboratory. Included in the parcel from Boverih Diamond Mines was 2.8 ct blue/ grey/green Pear and 1.9 ct green/grey Trilliant. On investigation, the cause of the colour in these two cases was entirely different.
Investigation of the 2.8 ct blue/grey/green diamond showed colour banding or zoning to be the source of the colour. Extending into the pavilion at a shallow angle to the girdle there was very obvious colour banding (Figure 2). This banding was visible in the body of the diamond under transmitted light, and on the surface under incidental light. There were several small patches of the original crystal surface remaining on the girdle and pavilion, and under transmitted light, patches of a brownish orange colour were observed on all of these areas of natural surface (Figures 3a and 3b). These brownish spotes are possibly radiation stains wich are common on the surface of any colour, including near colourless. While this was interesting it did not account for any colour that was present in thediamond.
Two of the crown mains showed a significant iridescence effect under incidental light, initially a possible indication of a coating (Figure 4). However testing with solvents and abrasives confirmed that the diamond was natural colour. Examination under transmitted light showed knot structures on five of the crown main facets and several of the crown girdle facets. These knots were closely related to the areas of iridescence effect on the crown mains (figure 5). The structure of the knots extends into the body of the diamond and was not confined to the surface, giving further proof that the diamond had not been treated.
Examination of the section of the diamond that showed banding confirmed this to be the cause of the colour. The banding consisted of bands of varying width and colour at a shallow angle to the girdle, and the diamond was cut to maximise the colour from the zoning. The predominant colours o the banding appeared to be blue and grey with occasional bands of grey/green colour and colourless material (See again figure 2). The shallow angle of the bands produced a slight but distinct directional change in the colour, with the colour depth being stronger on the side with the surface banding on the pavilion.This type of colour is strongly reminiscent of Hydrogen-rich diamonds.
The 1.9 ct green/grey Trilliant was a triangular shape with a simplified cut, consisting of only 15 crown facets (Including the table) and only 15 pavilion facets. Examination using a “diamond Sure” tester identified the diamond as a type IIa. Ulta violet examination at 235.4 nm and 365 nm showed the diamond to be inert. When placed under the microscope the cause of the green / grey colour was very obvious and extremely unusual. An extensive area of crystal surface remained in the pavilion (Figure 6) and the entire girdle consisted of the crystal surface (Figure 7), with the exception of one corner that consisted of a natural unpolished surface. All the areas of the original crystal surface had numerous green patches (Figure 8)which were identical in appearance with those described by Professor Wang et al (2004). These green patches are most probably irradiated stains due to the damage caused by alpha particles of U238 dissolved in underground water in contact with the diamond under formation. A very close examination of the colour patches under 100x magnification showed that the colour was on the surface only and did not extend into the body of the diamond. This is consistent with the fact that affect caused by alpha particles penetrates very little due to their high mass. The remainder of the diamond appeared to be nera colourless and would in all probability be EF in colour. Examination of the fractured area of the girdle showed no sign of any green coloration at all; however under transmitted light obvious epigenetic staining in the micro - fractures associated with the fracture was observed (Figure 9).
Apart from the fractures that gave the diamond a clarity grading of II, no structural features such as colour banding or graining were observed inside the diamond that could account for any of the colour visible. It would seem certain that further polishing of the remaining crystal skin would alter the green coloration from the diamond.
References
Professor E Hainschwang T., Massi L., Rondeau B. (2007) Hydrogen related optical
centers in natural diamond. Updated in New Diamond and Frontier Carbon Technology.
17 (2). Chapters 63 - 69
Professor Wang W., Lu T and Mose T.M. (2004) Lab Notes: Moon like surfaces on a
crystal. Gems and Gemology, 40 (2), Chapters 163-4.
CPayneRBauer. 2010. Gemmolgist. First Quarter 2010.
Bôverih. 1995. Full disclosure of natural diamond coloration. Chemistry department.
Presented to Smithsonian Institute.
Professor Shi Enci. An outline of International Diamond Grading.
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