October 3-7—JOSHUA TREE, CALIFORNIA: Annual show; Sportsmans Club of Joshua Tree
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 Post subject: HPHT diamonds
PostPosted: Wed Dec 30, 2009 1:29 pm 
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happy new year all... :smt002

I'm revising for my exams in Jan - trying to put in 8 hours a day - but its hard going. In the afternoon i try an attempt past papers...and i have a question. can anyone help?

HPHT treated diamonds - how would we determine that a diamond has been treated in this way. All i could think of is by using UV - Vis - NIR spectroscope......

Anybody got any other indications??The colours may change from brown to yellow or to colourless....but on observation after the treatment - how would we tell??

Thanks
P xxxx


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PostPosted: Wed Dec 30, 2009 6:52 pm 
Consluting my pre-FGA theory chart that I made myself, I have the following written for HPHT:

Quote:
Hpht 2a - Hazy, Internal Graining, FTIR, Raman, Black Crack Inc, Frosted Girdle, Inscription

Hpht 1 - Unnatural Yellow/Green, UV Green


It was a year ago that I memorised all this but, from memory, it depends if it is a Type 1 or Type 2A diamond.

The right candidate for most HPHT jobs is a type 2A stone (i.e. neglible nitrogen quantity) which is very clear of inclusions. Its discoloration (assumedly brown) should be due to internal structural deformations.

Essentially the HPHT's job is to force (using heat and pressure) the diamond's structure into something more 'normal' (less deformed), and thus remove the brown tint.

At these high temperatures, any inclusion in the stone might have differing thermal expansion to the diamond, and thus you might get a stress crack/halo.

Raman and FTIR would be, if memory serves, would also be detections of treatment at very high temperature.

Finally, an ethical HPHT treater might stick a inscription on the girdle. Even in these cases, a less ethical subsequent owner might remove this pretty easily.

As for Type 1 diamonds (contain nitrogen atoms - either in groups of three or individually dispersed), they tend to turn a very 'unnatural' yellowish-green with HPHT apparently. According to my chart thing, this should show up under UV as well.


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PostPosted: Sat Jan 02, 2010 6:49 am 
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Hi Keriakin...

Happy New Year... :D
Wow - you have an amasing memeory!
HPHT: Hazy, Internal Graining, FTIR, Raman, Black Crack Inc, Frosted Girdle, Inscription..............................what type of internal graining? I've never heard of Black Crack Inc!!! Why does the girdle frost?

I have another question too :D
I'm describing the refractometer etc...and pretty much get it all. OPtic character and sign and all the variations within uniaxial materials and how the 1 optic axes in uniaxial materials affects the results...depending on where it is with regards to the table facet etc.....Its written in the dip notes quite well. However - there is an exam question that asks me how the positions of the optic axes in a biaxial stones influence the results?? Which has thrown a spanner in the works!!! There's no mention of this in the notes...
I thought that light travelling down an optic axis remains unpolarised and unchanged = isotropic...... so if biaxial stones have two optic axes then the the chance of this happening increases? Is there anything else i need to know??

Ta
Pxxxx


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PostPosted: Sat Jan 02, 2010 7:15 am 
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Hi again - i have a real dumb question :oops:
Light propagating into a cubic stone (isotropic and SR) is it plane polarised as one single ray? Does SR material 'organise' the light? I'm supposing it does!!!! Hence the SR, otherwise it would be a mess.....like when traveling down an optic axes??!!!! :-k #-o :roll:

pxx


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PostPosted: Sat Jan 02, 2010 7:47 am 
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Hi again....i'm panicing now...i'm going through these past papers and certaind questions are coming up that go beyond the notes in the syllabus! A bit peeved!
Can someone enlighten me..

Question 1
How are brilliance and dispersion affected by changes to the proportions of the crown in a round brilliant cut diamond?
The smaller the table the smaller the brilliance. And the lower the crown the less fire....but other than that i don't know!!? I haven't done the diamond course?? :?

Then it goes on tto ask
Question 2
what benefits do you think a faceted girdle may make to the appearance of a cut diamond?? We haven't done that either!!! Its not in our notes!! I'm guessing: so that it increases brilliance and fire???

Pxx


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PostPosted: Sun Jan 10, 2010 7:38 pm 
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Pix,

If you have a the "ideal" crown angle of 31-37 degrees, you have a good balance of fire and brilliance, fire being dispersion - the splitting of white light into spectral colours, brilliance being the return of white light to the eye.

If you have a shallow crown you end up with a larger table which results in more brilliance and less fire, if you have a deep crown you have more fire less brilliance. In different parts of the world, people have different preferences for the balance of fire and brilliance.

Faceting the girdle will take away the white/grey appearance of a bruted girdle which can be reflected into the stone affecting the percieved colour, or causing nasty "fish eye" reflections particularly in stones with a shallow pavillion.

Hope this helps,

Kerry


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PostPosted: Sun Jan 10, 2010 11:01 pm 
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Pix....calm down, it's in your brain but you're letting panic blind you. Quit imagining gremlins where there are none....you'll do just fine!

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