THE ORANGE SAPPHIRE CONTROVERSY

Since late 2001 an influx of padparadscha (orange coloured) sapphires has occurred into the world market. It has been confirmed that these sapphires originated from the Chanthaburi gem market in Thailand, and that they are derived from Madagascan ruby and pink sapphire that has been colour enhanced by a still-secret heat treatment process. At the present time even commercial gem testing laboratories differ with respect to their hypotheses of how these attractively coloured sapphires have been produced. For example, several laboratories based in Thailand claim that the colour is simply the result of heat treatment. Other laboratories claim that the process is a heat diffusion process in which an ion foreign to the original chemical composition of the sapphire is deliberately diffused into the sapphire.

The problem gemmologist have to face is that these ?treated sapphires can display quite variable distributions of induced orange colour. Indeed three inter-related patterns of colour distribution have been recognised.

Over several months various laboratories have investigated the cause of this induced pink colour. Several hypotheses as to the probable cause of this induced colour have been suggested. Substitutional iron has been suggested as a possibly cause of this induced orange colour; but it has been shown the concentration of iron in these sapphires is insufficient to support this hypothesis.

Fortunately, advanced chemical analysis initiated by GIA researchers has revealed the most likely cause of the induced orange overtone in these sapphires. 

This research has clearly indicated that very small amounts (only several parts per million) of beryllium ions (Be²⁺) have been heat diffused into these sapphires to create an orange colour producing Trapped Hole Colour Centre or THCC. This particular THCC is a point defect in the sapphire’s crystal structure where charge balance does not occur when an impurity Be²⁺ ion replaces an Al ³⁺ ion. If the sapphire is heated under highly oxidising conditions - that is in the presence of excess oxygen – a ‘hole’ (loss of an electron) is created on one of the six pairs of oxygens that surround each Be²⁺ ion. This hole, which has a single positive charge (i.e. only one electron), combines with the Be²⁺ to provide the necessary +3 charge at this site. 

It is a fact that heat controlled diffusion rates for ions such as Cr³⁺, Fe³⁺ and V³⁺ in corundum are very, very slow. In contrast, ions with differing valencies, such as Mg²⁺, Ti⁴⁺, and indeed Be²⁺ will diffuse into corundum some many thousand of times faster - and to greater depths unless they are complexed (halted) by other ions in their path. As a consequence, and depending on the conditions of the oxidative heating, Be²⁺ can be made to penetrate either to shallow depths, completely throughout the sapphire, or to intermediate depths between and so create the THCCs responsible for a yellow colour. Subsequent heat treatment can, of course, reverse this process, leaving behind an orange core and a pink outer rim.

The problem that gemmologists currently face is how to possibly identify this treatment. Unfortunately, at the present time no criteria exist to facilitate this identification.

So what can the gemmologist do?

The answer is very little… so BE CAUTIOUS … and REFER ANY SUSPECT ORANGE SAPPHIRE TO A COMPETENT LABORATORY FOR AN EXPERT OPINION.

If you are requested to examine an orange to orangey pink sapphire, then the following steps are recommended:

1. Examine the sapphire table-down on a white surface, noting whether the
   sapphire displays an orangey ‘halo’ around its girdle.
   Implication: If this halo is noted, the sapphire is likely a heat diffusion treated.
   

2. Examine the sapphire, immersed, in transmitted white light for evidence of:
   
   … an orange coloured rim that can have a variable thickness, its depth
   may vary over the sapphire, and its junction with the pink core is not well
   defined
   Implication: Likely a heat diffused treated.
   … an orange core and a pink rim.
   Implication: Could be a heat diffused sapphire that has been additionally
   heat-treated in a reducing atmosphere.
   

3. Examine the interior of the sapphire, under magnification, for evidence of
   heat treatment.
   Implication: The presence of melted solid inclusions, induced fracture halos
   etc. only confirms heat treatment.

Note: The accentuated facet edges, thin well defined blue coating, and range of colour between facets that typify diffusion coated blue sapphires¾and will not be noted on these diffusion treated orange sapphires. 

In a report, posted on the internet on the 25th February 2002, the unified international gemmological laboratories of the AGTA, the GIA, and the Swiss Gübelin and SSEF laboratories agreed that orange sapphires from Thailand that display the features specified above should be described as:

Treated (orange) sapphire, with indications of heating, in which the orange coloration is confined to a surface-related layer

… in spite of the obvious fact that these sapphires display most of the features of a
surface diffused treated sapphire.

Unfortunately, at this time, the major commercial gem testing laboratories can’t be sure of the precise mechanism being used to colour enhance these sapphires; hence the caution expressed in their description of this new treatment..

This is where the ‘state of play’ with respect to these new treated orange sapphires lies in the early months of 2002. 

Readers interested in further information on this 
topic should consult the following websites:

• AGTA GTL Orange pink sapphire alert of Tuesday January 8th on the AGTA website

• Pala International News of February 16th on the Pala International website

• GIA INSIDER of Friday February 15th on the GIA website.

• GIA internet release of 25th February 2002.

 

Grahame Brown
Editor
The Australian Gemmologist

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