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? about Pavilion facets and LGFs, kind of techy

Kalrik

Shiny_Rock
Joined
May 15, 2011
Messages
143
I recently purchased a diamond from JA. It had a very good HCA score and IS image, but I feel it had an obstruction issue. I have black, long hair, and I felt like I couldn't look at the diamond without seeing it dark in the center (ok this is an exaggeration, but it occured frequently). Here are its numbers:

Crown angle: 35
Pavilion angle: 40.8
LGF: 75% GIA certed (this was the issue?)
total depth: 61.4
table: 58
star length: 50
girdle: medium to slightly thick faceted.

It was a very sparkly and firey stone, however.

Could I eliminate the problem of obstruction with a steeper pavilion angle and shallower crown angle only? Or must I get longer LGF? For example, I am considering this stone which has the same LGF but looks quite different:

http://www.jamesallen.com/diamonds/J-SI2-Ideal-Cut-Round-Diamond-1378778.asp

What accounts for the larger white area in the center of this diamond (the one above) vs. the one below, for example? (Sorry I cannot copy or save their images) Something with the pavilion main facets? Why do the arrows seem relatively thin (more space between them / not as fat as most LGF 75% I have seen on the virtual loupe-like the one below) for a 75%? Is it because of the larger white center area? How will this white area in the center perform? Also, I just wanted to mention both these stones have 75% LGF but yet look different.

http://www.jamesallen.com/diamonds/D-VVS2-Ideal-Cut-Round-Diamond-1317083.asp

Sorry for all the questions, if it's too much to sift through and answer here is the MAIN QUESTION: I know there is a lot of rounding here on the GIA reports, but what I am trying to ask is aside from the actual LGF percentage, what affects the thickness of the arrows under the table (ie: something with the shape of the pavilion main facets? is the effect from the pavilion angle large-41 degrees with 75%will look different than a 40.6 degrees with LGF 75%?).

I know there are a lot of threads on LGFs, but I couldn't find the answer to my exact question. Hopefully someone will understand my poor writing skills and newbie way of describing everything.
Thanks for getting through this long post and for your help!
 

Stone-cold11

Super_Ideal_Rock
Joined
Sep 9, 2008
Messages
14,069
Center white is table reflection. Larger table reflection is generally an indication of relatively deep pavilion compare to crown.

Shaft size, maybe partly rounding difference partly optical illusion, the large table reflection covering up the base of the arrow shafts.

How close are you looking at the stone? That stone should not have obstruction issue unless there is something wrong with UD's idealscope setup. All stones will have some black but arrows are not easily seen until you are looking at it less than 1 foot away.
 

Lula

Ideal_Rock
Joined
Apr 5, 2009
Messages
4,544
Stone-cold11|1307929020|2944389 said:
Center white is table reflection. Larger table reflection is generally an indication of relatively deep pavilion compare to crown.

Shaft size, maybe partly rounding difference partly optical illusion, the large table reflection covering up the base of the arrow shafts.

How close are you looking at the stone? That stone should not have obstruction issue unless there is something wrong with UD's idealscope setup. All stones will have some black but arrows are not easily seen until you are looking at it less than 1 foot away.
I agree with the part of stone-cold's reply that I bolded above re the effect of the table size on your perception of the arrows.

Here is a graphic (posted previously by John Pollard) that depicts the effect of table size on the face-up appearance of a diamond (the pavilion angle is held constant at 40.75 and crown angle held constant at 34.5 in the top two images). Note that the larger table on the right (62% table) creates dark areas in the center of the diamond. This darkness is not leakage (if the stone is cut well, that is); it is table reflection, and this may be what your eyes see as darkness near the center of the stone. It is normal. Many people like diamonds with large tables because the larger table gives the diamond a different "look." Others prefer a smaller table. As long as the diamond's angles work together properly (i.e., its proportions were chosen to maximize beauty not carat weight) the stone will sparkle and not exhibit leakage.

table-reflection-forps.jpg
 

Lula

Ideal_Rock
Joined
Apr 5, 2009
Messages
4,544
Here's a photo of a beautiful ring posted by PS member didiamond. This is a very large stone (I forget the exact carat weight) but this photo shows clearly the effect of a larger table on the appearance of the stone -- the center stone shows a larger white "circle" in the center with some darker areas surrounding the "circle". Is this similar to what you are seeing as darkness?

closeupRB - didiamond.jpg
 

Kalrik

Shiny_Rock
Joined
May 15, 2011
Messages
143
Thanks for the responses, really was surprised to get any. The table reflection nails the question I had about the white center. Thanks for the images too.

In that last picture, no that is not what I saw. It was thick, dark triangles radiating from the center (the arrows), but they also appeared to lose some of the space between them to darkness like they had in the IS image (think a cartoon sun=dark parts i see with a white circle in the center). I know what I had was a beautiful diamond, but I think for my Personal tastes I prefer long LGFs and shallower crowns ( I saw one with a very shallow crown in the store and just loved it). Again, thanks for going through the work of reading that OP and attaching the images, cleared up my questions! =)
 
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