Classification of brown diamonds and their color origin - GemNantes

CO2 may be present in these stones in another phase or form than in the CO2
diamonds; it has been shown by HPHT experiments on liquid CO2, that various solid
phases can be produced, some of which fairly stable at ambient conditions (seehttp://www.llnl.gov/str/Yoo.html).
Our temperature/pressure of 2000°C and 6.5 GPa (65 kilobar) would suggest the
formation of liquid CO2 during the treatment, which does not explain the findings of
the formation of the 2390 cm -1 peak by HPHT annealing of the “Pseudo-CO2”
diamond.
8. Conclusions
The results of this classification and characterization of brown diamonds were
unexpected looking back before our analysis; the variety of brown diamonds is
highly complex and the color formation not easy to understand. The HPHT annealing
experiment was helpful to clear up some points, but others still remain mysterious; a
good example is the main “amber center” peak, which can be found at different
positions, depending on type and/or color; the HPHT experiment has shown that all
these absorptions are not very temperature stable. They were eliminated in all cases
by annealing at 2000°C at 6.5 GPa stabilizing pressure; this was always accompanied
by a strong reduction of the platelet peak and some hydrogen features. The amber
center peak at 4165 cm -1 has been found to be related to the presence of A-
aggregates and the ratio of A vs. B aggregates; besides this, a shifted amber center
absorption in “regular” Ib brown diamonds has been found at 4115 cm -1 which
appears to be correlated to the single nitrogen content; and a side peak at
4070 cm -1 was found to be related to olive coloration and to the presence of
nitrogen, but not to the nitrogen aggregation state; this peak has been found in all
type I diamonds with an olivish brown to olive color; it may be related to the C-
centers since most of these diamonds, despite having type Ia IR spectra, show a Ib
character; this can be seen in their PL spectra, where strong NV - related emissions
(e.g. at 637 nm) can often be detected.
Looking at these facts, it can be postulated, that the amber center is a defect
directly related to the A-aggregates and to the defect, which causes the brown
coloration in diamonds. If indeed amorphous carbon is responsible for the brown
color, this defect may well be due to an “aggregate” consisting of the A-aggregates
and amorphous carbon; the shifted peak at 4115 cm -1 seen in “regular” type Ib
browns appears thus to be related to N 0 , and, if the amorphous carbon theory is
correct, may be an “aggregate” consisting of single nitrogen and amorphous
carbon. Since no defect in this region has been found in type IIa brown diamonds,
this nitrogen-dependence must be assumed. No amber center was present in the
few pure type IaB brown diamonds thus it can also be proposed that the B-
aggregates do not form an IR active defect with the defect responsible for the
brown coloration.
The observed decrease of the 4165 cm -1 peak in all near-pure type IaB diamonds
supports this idea. Thus the amber center defect at 4165 cm -1 appears to be
dependent on the presence of A-aggregates and no strongly dominating B-
aggregates while the C-centers in regular brown diamonds and some olive stones of
type Ib appear to cause a shifted amber center at 4115 cm -1 .
The lack of an “amber center” in some brown diamonds exhibiting regular colored
graining can be explained as follows:
Type IIa • no or nearly no nitrogen present.
Type IaB pure• B-aggregates are not involved in the formation of the amber center.
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