What causes this dead space in the center of a round?

[MMtwo]

I a curious to learn why some diamonds have a larger center that seems like dead space right before the facets. This diamond is a great example of a large one (whatever it is)...In an old cut, it would be the culet area. Is there a name for it? How do I avoid it?

Thanks!

PS: Don't worry, this stone is not under consideration!

 

[the_mother_thing]

I could be mistaken (and hope - if I am - that someone corrects me), but I think it’s a product of the table/its reflection, the appearance of which is impacted by other cut parameters.

@Karl_K Can you weigh in?

 

[MMtwo]

The parameters for this stone:



The fact this is an EX stone amazes me. I don't find it remotely attractive.

 

[Karl_K]

table size plus very deep pavilion and lgf%.
It is the area between the end of the lgf and the culet.
It exists on all RB diamonds but is larger and more prominent with a larger table or deeper pavilion and varies with lgf%.
Smaller with longer lgf%, larger with short lgf%
60% table and a 41.6 pavilion with a on the short side lgf% makes it large and stand out more in this case.

 

[MMtwo]

table size plus very deep pavilion and lgf%.
It is the area between the end of the lgf and the culet.
It exists on all RB diamonds but is larger and more prominent with a larger table or deeper pavilion and varies with lgf%.
Smaller with longer lgf%, larger with short lgf%
60% table and a 41.6 pavilion with a on the short side lgf% makes it large and stand out more in this case.

Thank you for the explanation @Karl_K ! very helpful.
Thanks @the_mother_thing for tagging Karl and your help.

 

[Rhino]

Yes table reflection.

Here's 56 alongside of 60% tables. Changing the lower half facet length doesn't alter the size of that reflection but a deeper pavilion will absolutely make it larger.

 

[MMtwo]

@Rhino Thanks for the graphs, I can see the reflections now. Very helpful.

 

[Garry H (Cut Nut)]

At 42 degree pavilion angle the pavilion depth % is 50%. It just so happens that this number lines up nicely and the table reflection measure 50% of the way from culet to corner of table facet. See my red lines.
At 45 degree pavilion angle the depth AND the table reflection are both 100%. i.e. any light entering perpendicular to the pavilion will exit at the exact same angle (from pavilion main facets).
At 38.66 degrees there is no table reflection at all. This corresponds to a 40% pavilion depth. So you can use this to roughly indicate the pavilion angle.
For example at 43.3% depth the table reflection takes up 1/3rd of the table region.

 

[Garry H (Cut Nut)]

The light that falls inside the table reflection goes out the table.


The light that falls in the outer area of the table reflection leaves via the opposite side crown facets.

 

[MMtwo]

The light that falls inside the table reflection goes out the table.


The light that falls in the outer area of the table reflection leaves via the opposite side crown facets.

I think I followed this, but am not quite sure. It took a cup of coffee before my first attempt. I'm on my second cup of coffee and chewing on the diagrams. I appreciate the depth of information. Thank you. By the way, I have enjoyed your website with the HCA details and breakdown at http://www.diamond-cut.com.au/.

 

[Garry H (Cut Nut)]

 

[MMtwo]

@Garry H (Cut Nut) okay, the lightbulb went on now. Thank you for taking the time to try again! Very helpful to understand how and why that happens.