About the platelet Apeak bout the platelet peak of HPHT- treated odiamonds f HPHT- treated diamonds of otype f type IIa a
a presentation by Jean-Pierre Chalain
© SSEF, May 2009
About the platelet peak of HPHT- treated diamonds of Atype bout the platelet peak of HPHT- treated diamonds of type IIa a
– platelets & platelets peak
– HPHT treatment of type I a diamonds
2. The width of the platelets peak
– before & after HPHT treatment
– for greenish yellow diamond of natural colour
3. FW (B’) natural vs H PHT treated
EGS, Bern 2009 - SSEF, JPC
European Gemmological Symposium, Bern 2009SSEF, JPC
1. Background:1. Background: P P l ateletsl atelets
General definition:General definition: platelet: n oun P hysiologya
small colourless disk-shaped cell fragment without a nucleus, found in large numbers in blood and involved in clotting (coagulation).
Also called thrombocyte.
EGS, Bern 2009 - SSEF, JPC
European Gemmological Symposium, Bern 2009SSEF, JPC
Definition: Definition: ‘‘P latelets in diamondsP latelets in diamonds’’
Layers of interstitial carbon atoms. They were first observed by Evans & P hal in 1962 by means of a Transmission Electron Microscope.
Platelets lay in the 001 crystallographic diamond plan. Platelets are only present in t ype Ia diamonds containing A and B aggregates or B aggregates only. Platelets are by product of nitrogen aggregation (from A to B) and of the formation of the N3 optical centre. Some platelets are of the order of several tens of nanometres and larger ones may attain 1 micrometer.
EGS, Bern 2009 - SSEF, JPC
European Gemmological Symposium, Bern 2009SSEF, JPC
1. Background:1. Background: P P l atelet peakl atelet peak
The platelet peak appears in the infrared absorption spectrum of a type Ia d iamond
B aggregates A aggregates
Platelet peak (1375 cm 1 )
The position of the platelet peak (B’ ) ranges from 1358 cm-1 to 1378 cm -1 .
The higher the absorption coefficient, the higher the concentration of platelets.
EGS, Bern 2009 - SSEF, JPC
European Gemmological Symposium, Bern 2009SSEF, JPC
1. Background: HPHT treated diamonds of type1. Background: HPHT treated diamonds of type I aI a
Some conditions for HPHT treatments:
•Pressure: 60-75 kBar
•Temperature: 1800°C - 2100°C
•Duration: 4 to 50 hours (or more)
There are plenty of valuable publications for describing the HPHT treatment. But there are very few publication on natural greenish yellow diamonds. Therefore, t he separation of greenish yellow diamonds of natural colour from those HPHT treated is sometimes difficult.
EGS, Bern 2009 - SSEF, JPC
European Gemmological Symposium, Bern 2009SSEF, JPC
1. Background: HPHT treated diamonds of type1. Background: HPHT treated diamonds of type I aI a
The H 5222 rough diamond (1.11 ct) belongs to the Natural Historical Museum of Vienna since 1907.
This diamond is a perfect example of a greenish yellow diamond of natural colour and it shows similar optical centres to those seen in HPHT treated diamonds (Chalain & al, 2005).
Here, we introduce a new criterion that will ease the separation between greenish yellow diamonds of natural colour and HPHT treated greenish yellow diamonds.
EGS, Bern 2009 - SSEF, JPC
European Gemmological Symposium, Bern 2009SSEF, JPC
2. The platelet peak before and after HPHT treatment2. The platelet peak before and after HPHT treatment
When comparing the width of the platelet peak before and after a HPHT treatment we observed that after treatment the platelet peak is both reduced and broadened
EGS, Bern 2009 - SSEF, JPC
European Gemmological Symposium, Bern 2009SSEF, JPC
2. FW (B2. FW (B’’) for natural greenish yellow diamond) for natural greenish yellow diamond
From this last observation, we decided to check the width of B’ for untreated greenish yellow
In 2005 the width of the H5222 was the first published value (Chalain & al 2005)
The platelet peak centred at 1 360.7 cm-1 has an absorption coefficient of 2.3 cm-1 and its FW is 7.3 cm-1.
EGS, Bern 2009 - SSEF, JPC
European Gemmological Symposium, Bern 2009SSEF, JPC
3. Comparison of the platelet peak
3. Comparison of the platelet peak
’’s width for untreated and HPHT treated diamondss width for untreated and HPHT treated diamonds
DTC Research Centre and SSEF have plotted on the same graph, the widths of platelet peaks
EGS, Bern 2009 - SSEF, JPC
European Gemmological Symposium, Bern 2009SSEF, JPC
Why is the platelet peak modified after a HPHT treatment?
1. It is reduced in intensity, this means that the HPHT treatment reduces the concentration of interstitial carbon atoms.
2. We know that the HPHT treatment releases a large quantity of carbon vacancies (those responsible for the original brown colour of the diamond)
3. There is most probably a C-V recombination where carbon atoms reoccupy their normal lattice site
Why does the platelet peak broaden after a HPHT treatment?
As already mentioned in the case of the broadening of the NV negative photoluminescence peak stress is involved in the broadening of the platelet peak during the HPHT treatment
What about the area of uncertainty?
Further research is needed, especially for taking in consideration several factors that increase the width of the platelet peak (total absorption at 1282 cm-1, (Woods, 1986), regular or irregular diamonds, etc.)
EGS, Bern 2009 - SSEF, JPC
European Gemmological Symposium, Bern 2009SSEF, JPC
We have shown that the width of the platelet peak is good indicator for separating (1) greenish yellow diamonds of natural colour and (2) HPHT treated diamonds of the same colour.
This, so far, unpublished criterion added to already published criteria will considerably increase the certainty in separating these two families of diamonds.
I thank Dr. David Fisher, DTC Research Centre, Maidenhead for his help and for sharing his data with SSEF.
I also thank Dr. Benjamin R ondeau f or his support.
I’ll be happy to answer any possible question.
EGS, Bern 2009 - SSEF, JPC
European Gemmological Symposium, Bern 2009SSEF, JPC
Thank you for your attention