How to read this Star sapphire NGI grading report?

[chictomato]

Hi there I have just graded this star sapphire and was told that it is of quite a good grade, however I do see silk on the stone, it does not seem clean and clear to me. Do take a look at the report and let me have you views on this star sapphire grade and approximate cost in the market? tks

Natural White Star Sapphire
(Corundum)

3.79carats

10.69X8.69X4.11mm

Oval, Cabochon

Translucent

Refractive Index 1.76 (spot reading)

Specific Gravity 3.99

Fingerprint, silk and growth zoning

Other test: Pleochroism and fluorescence

Comments: No indications of thermal enhancement. This stone display an asterism. Gemological testing revealed characteristics consistent with those of sapphire originating from Sri lanka.

Nan Yang Gemological Institute

 

[Indylady]

Most cabs (esp. those more expensive gems) aren''t very clear. If they were, they probably would have been cut. I think the silk is part of its charm.

 

[morecarats]

In star sapphires there is always a tradeoff between clarity and asterism. Because the tiny inclusions of rutile or titanium dioxide (aka silk) are responsible for asterism in the stone, you want to have enough rutile for the star, but not too much. Nature being what it is, most star sapphires are cloudy to opaque. Translucency is usually the most you can hope for; only exceptional specimens are really transparent. It is also worth noting that many star sapphires are diffusion treated with titanium dioxide to improve the star. Of course the treatment tends to make them less transparent as well.

 

[chictomato]

Date: 11/29/2009 12:04:32 AM
Author: morecarats
In star sapphires there is always a tradeoff between clarity and asterism. Because the tiny inclusions of rutile or titanium dioxide (aka silk) are responsible for asterism in the stone, you want to have enough rutile for the star, but not too much. Nature being what it is, most star sapphires are cloudy to opaque. Translucency is usually the most you can hope for; only exceptional specimens are really transparent. It is also worth noting that many star sapphires are diffusion treated with titanium dioxide to improve the star. Of course the treatment tends to make them less transparent as well.

Is diffusion treated with titanium dioxide considered as thermal treatment? will it be reflected in a report? base on the info on the sapphire, is it of a good grade? how much will this cost approximately? tks

 

[morecarats]

Date: 11/29/2009 3:23:34 AM
Author: chictomato

Date: 11/29/2009 12:04:32 AM
Author: morecarats
In star sapphires there is always a tradeoff between clarity and asterism. Because the tiny inclusions of rutile or titanium dioxide (aka silk) are responsible for asterism in the stone, you want to have enough rutile for the star, but not too much. Nature being what it is, most star sapphires are cloudy to opaque. Translucency is usually the most you can hope for; only exceptional specimens are really transparent. It is also worth noting that many star sapphires are diffusion treated with titanium dioxide to improve the star. Of course the treatment tends to make them less transparent as well.

Is diffusion treated with titanium dioxide considered as thermal treatment? will it be reflected in a report? base on the info on the sapphire, is it of a good grade? how much will this cost approximately? tks

Yes, this sort of diffusion treatment would be considered a form of thermal treatment, since heat is required to achieve the diffusion. The lab that issued the report is a respectable lab in Singapore, so I would be inclined to trust their testing if they included no evidence of heat treatment.

I don''t think you''ll get useful responses on the value of the gemstone here, certainly not without photographs. The lab that issued the report on the stone also does financial appraisals, so it might be best to seek their opinion.

 

[chictomato]

hi thanks alot morecarats, here is the link to all the photos star sapphire. Mr Zeng said that they do not provide valuation of gems, all he can say is that its of a good grade. pls let me know your views!

 

[chrono]

So the star sapphire came back as untreated? That's great. You paid a good price on it. Of course, if it were more transparent or less gray, it'll be worth more, but then again, wouldn't a lot of other gemstones too?

 

[chictomato]

tks morecarats, szh! Tks chrono, you have always been helpful!