Question regarding GIA cut and holloway

[kath1969]

I am looking at 3 GIA certified diamonds. Two are "excellent" cut and one is "very good" cut. Interestingly, I ran them through the holloway cut advisor. The two excellent cut diamonds scored a 4 while the very good cut diamond scored under 2. Is this possible?

 

[stone-cold11]

Yap, it is possible.

Give us the numbers and we will explain to you why it is so for each case.

 

[Garry H (Cut Nut)]

Date: 4/25/2009 8:26:31 PM
Author: Stone-cold11
Yap, it is possible.

Give us the numbers and we will explain to you why it is so for each case.

Thanks StoneCold, also remeber the VG can be downgraded for other reasons like girdle sym and polish

 

[kath1969]

The first two:

1.06 G SI1
6.46 6.49 4.03
Cut: Excellent
Polish: Excellent
Symmetry: Excellent
Depth 62.2
Table 58
Crown 35.5
Pavilion 41
Girdle Med to Slightly Thick


1.09 G VS2
6.58 6.63 4.08
Cut: Excellent
Polish: Very Good
Symmetry: Very Good
Depth 61.8
Table 58
Crown 35.5
Pavilion 41.2
Girdle thin to slightly thick


The last one:

1.00 G SI1
6.33 6.44 3.95
Cut: Very Good
Polish: Excellent
Symmetry: Very Good
Depth 61.9
Table 57
Crown 36.5
Pavilion 40.4
Girdle medium to slightly thick

 

[stone-cold11]

The first two have a steeper pavilion angles for the respective crown angles, this will result in light leakage. If you look at the graph of the HCA for each table size, the green dot outline represents what GIA defines as it''s Ex cut candidates. That also includes a large amount of green and blue, which are less than ideal in performance.

For the third stone, it''s angles are outside what GIA considered as Ex cut, but AGS will consider it Ideal cut, HCA also consider it a good candidate for further investigations. Just different range of angles that each lab consider as Ex/Ideal in this case.

For us, I would consider #3 stone further, for example asking for an IS/ASET image to see if I like the light return and symm, checking if eye-clean, etc...

Hope that answer your question.

 

[Lorelei]

The first two diamonds are actually what we call steep deep, this means both crown and paviilion angles are not a good fit for each other. This can result in light leakage with these types of diamond. These angle combos can get the GIA Excellent cut grade unfortunately and can routinely score badly on the HCA. You can do better.

The third looks the most promising so if you could get ASET or Idealscope that would be helpful then we can check it out for you. Can you get these images?

The third diamond will probably got VG cut due to the pavilion angle.

 

[kath1969]

Thank you so much for the replies. The third diamond is the least $''s because it is slightly smaller and rated a very good cut. I haven''t viewed the diamonds yet, but it will be interesting to see them side by side and see if I can pick up on the difference.

 

[Lorelei]

Let us know how you get on.