Carat vs Dimension

[stitch]

I am always of how the experts can look at the dimension of a diamond RB and determine the face up look of whether it looks more or less than its supposed carat weight. How do we determine the look of a the diamond based on dimensions? what is the correlation?

Thanks

 

[kenny]

Spread is the diameter for a given weight.
Deep diamonds have poor spread.
Shallow ones have good spread, or face up large for their weight.

But what matters much more than spread is good cut.
A well cut diamond returns more light and will therefore look larger than a poorly cut one of the exact same diameter.

I say forget about spread and focus on cut quality.

Looking for a 1 carat of a particular diameter may not give you good cut because the girdle may be too close to, or far from, the table.
See example of this in my next post.

 

[kenny]

Here.
Notice they have the same diameter but the girdle position is different.

 

[stitch]

I see. How do we calculate and determine a good cut without using HCA. A typical scenario is we are at a B&M and look at a diamond. At that point, we only have a calculator on the phone and the diamond report with dimensions provided.

 

[tonyc2387]

lorelei came up with a good cheat sheet that''s reproduced pretty frequently around here... it''s a great thing to go by, just write these numbers down:

depth - 60 - 62% - although my personal preference is to allow up to 62.4%
table - 54- 57%
crown angle - 34- 35 degrees
pavilion angle - 40.6- 41 degrees
girdle - avoid extremes, look for thin to slightly thick, thin to medium etc
polish and symmetry - very good and above

thanks lorelei!

 

[Lorelei]

Thanks Tony!

Stitch with well cut diamonds normally the spread will fall into place as a natural result of good planning and cutting of the diamond, however the way to check it is to know that the diameter measurement in round brilliants is the spread measurement and that certain weights in rounds should fall within certain spreads. For example a .80 ct should spread around 6 mm, 1 carat 6.5 mm, 1.5ct 7.5mm and so on. There are charts here you can use or the in house cut tool has appropriate spreads for the weight within that search.

 

[denverappraiser]

Date: 12/13/2009 6:08:19 PM
Author: stitch
I see. How do we calculate and determine a good cut without using HCA. A typical scenario is we are at a B&M and look at a diamond. At that point, we only have a calculator on the phone and the diamond report with dimensions provided.

A good start is to ask to see ONLY diamonds that have been graded by GIA or AGSL. Assuming you are looking at round diamonds, they both contain a cut grade on the lab report.

Neil Beaty
GG(GIA) ICGA(AGS) NAJA
Professional Appraisals in Denver

 

[stitch]

tonyc2387,

Thanks for providing that reference.

Lorelei & denverappraiser,

We are going to shop a diamond for my friend at a B&M for for an RB for my friend. Providing my knowledge on diamond''s spread is next to nothing, I am kinda scare. This B&M just told my friend that he is going to have a diamond imported from Belgium and graded by GIA comes in this Tuesday. He told me that he will provide the cert number today. He provides a basis infor on this stone

Round Brilliance

Carat : 1.32
Color : H
Clarity : IF
Price : 8100.

This is all that we got for now. I think the price is kinda low. Do you think H will show tint when viewing from the side?

 

[oldminer]

Here is the AGA Cut Class chart for rounds. There are "rules" for overall grading a stone on www.gemappraisers.com, but you can use these individual characteristics to assist you. Shoot for the best cuts at 1A and 1B and Very good cuts in the 2A and sometimes 2B range. It is a screening too, but it can be very useful.

 

[stitch]

Hi David,

Can you provide the link to chart please. I couldn''t read the chart in the attached image

thanks

 

[stone-cold11]

http://www.gemappraisers.com/oldcutgraderules.asp

 

[stitch]

Thanks stone-cold