about the platelet peak of hpht- treated diamonds of type iabefore hpht after hpht there are plenty...

a presentation by

Jean-Pierre Chalain

© SSEF, May 2009

About the platelet peakAbout the platelet peak

of HPHT- treated diamondsof HPHT- treated diamonds

of type of type IaIa

European Gemmological Symposium, Bern 2009 -

SSEF, JPC

EGS, Bern 2009 - SSEF, JPC

LayoutLayout1. Background

– platelets & platelets peak

– HPHT treatment of type Ia diamonds

2. The width of the platelets peak– before & after HPHT treatment

– for greenish yellow diamond of natural colour

3. FW (B’) natural vs HPHT treated

4. Discussion

5. Conclusion

About the platelet peak of HPHT- treated diamonds of type About the platelet peak of HPHT- treated diamonds of type IaIa


SSEF, JPC


General definition:General definition:

platelet: noun Physiologyasmall colourless disk-shaped cell fragment without a nucleus, found in largenumbers in blood and involved in clotting (coagulation).Also called thrombocyte.

1. Background:1. Background: P Plateletslatelets


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1. Background:1. Background: P Plateletslatelets

Definition: Definition: ‘‘Platelets in diamondsPlatelets in diamonds’’Layers of interstitial carbon atoms.

They were first observed by Evans & Phal in 1962 by means of a Transmission Electron Microscope.

Platelets lay in the 001 crystallographic diamond plan.

Platelets are only present in type 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.


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The platelet peak appears in the infrared absorption spectrum of a type Ia diamond

Platelet peak

(1375 cm-1)

B aggregates

A aggregates

1. Background:1. Background: P Platelet peaklatelet peak

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.


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1. Background: HPHT treated diamonds of type 1. Background: HPHT treated diamonds of type IaIa

Some conditions for HPHT treatments:

•Pressure: 60-75 kBar

•Temperature: 1800°C - 2100°C

•Duration: 4 to 50 hours (or more)

Before HPHT After HPHT

There are plenty of valuable publications for describing the HPHT treatment.

But there are very few publication on natural greenish yellow diamonds.

Therefore, the separation of greenish yellow diamonds of natural colour from those HPHT

treated is sometimes difficult.


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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.

1. Background: HPHT treated diamonds of type 1. Background: HPHT treated diamonds of type IaIa


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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


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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 1360.7 cm-1 has an absorption coefficient of 2.3 cm-1 and

its FW is 7.3 cm-1.

H 5222


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3. Comparison of the platelet peak3. 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


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4. Discussion4. Discussion

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

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.)

What about the area of uncertainty?


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5. Conclusion5. Conclusion

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.

AcknowledgementsAcknowledgements

Thank you for your attention

I thank Dr. David Fisher, DTC Research Centre, Maidenhead

for his help and for sharing his data with SSEF.

I also thank Dr. Benjamin Rondeau for his support.

I’ll be happy to answer any possible question.

about the platelet peak of hpht- treated diamonds of type iabefore hpht after hpht there are plenty...

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