Spread index - Acceptable range?

[drdeano]

Hi there! I''ve been a lurker for a few weeks and would like to see if I might get some clarification about a stone I just purchased.




Stone Specs:


GIA Cert.
Round Brilliant
6.00-6.05 x 3.74
.84 carat
Depth - 62.1%
Table - 58%
Girdle - Medium to Slightly thick, Faceted
Culet - None
Polish - EX
Symmetry - VG
Clarity - VS1
Color - F
Fluor - None




Let me first say, this stone is beautiful. It sits in the center of a truffle platinum setting with two GIA ideal cut F .33 carat rounds.




I realize the center does not fall within "ideal" guidelines as the table percentage is larger than 57%. Although I am taken aback by the beauty of the stone, and the light performance seems breathtaking to me....I can''t help but wonder if I couldn''t have done better! I ordered an idealscope yesterday so I could see how it performs on that scale, but after doing more reading I''m more confused than ever. In particular, I am concerned about the spread of the stone. I ran across a few threads where people were mentioning their 3/4 carat stones which had a 6.0 diameter.. and it made me think..that is how big my .84 is!!




Since the stone is GIA, obviously I''m missing some numbers that could help me figure this all out mathematically (or I could just be less anal!) I used the "spread index" calculation to see how well (or not) my stone measured up.




Spread Index = (average diameter)/(carat weight)^(1/3) / 6.5


My .84 stone = (6.025)/((.84)^(1/3)) = 6.386mm rescaled to 1 carat


Indexed at 6.386/6.5 = .982




Assuming the closer to 1 the better?


After doing all this, I needed a reference. I checked some stones on the GOG and Superbcert sites and found those stones to be almost exactly 1. My question is; how far off is acceptable? Does the spread of this stone mean I paid for a .84 when I really got a .80 if it were cut perfectly??




I think I''m a victim of wanting to know TOO much when I should just shut up and be happy. It is a beautiful stone, but I''m a numbers guy.. and I want to know I did pretty good for someone I love and want to spend the rest of my life with. Anyone have any info they could share? Maybe someone with DiamCalc could plug in some numbers and give me the news? Is my stone cut too deep? Any info would be greatly appreciated!




Great site! Great folks!


-DrD

 

[Demelza]

Hi DrD,

I think one of the most important things you said is that the stone is "breathtaking." I know it's hard not to get caught up in the numbers (goodness knows, I have), but it sounds like this stone really "speaks" to you. As for the diameter, it sounds like it might be just a bit on the small side for the carat weight. I would imagine some of the weight is in the "slightly thick" girdle. From what I've seen, the spread of your diamond is more like that of an ideal cut .80 which tend to be around 6 mm. An ideal cut .84 might be around 6.10 mm. Will a tenth of a mm (or less) make a difference? You might be able to notice if two stones are side by side, but I can't imagine you would notice once the ring is on her finger. The most important thing, though, is for you to feel comfortable with the decision. If you think it will bother you knowing that you didn't get that extra tenth of a mm, then by all means, find another stone. But it sure does sound like you love the one you've already found.

 

[Garry H (Cut Nut)]

The stone has about .05ct of excess weight and that will reduce its ideal-scope light return.

It is not possible to have the best combination of proportions.
I suspect you are examining the stone loose and light is able to enter the pavilion and you can see this additional light that would otherwise be leakage in the table area and around the outer girdle.
You might also be in a store with radioactive lighting

 

[Superidealist]

I prefer to define spread index as:

Spread Index = (average diameter)/(carat weight)^(1/3)

The formula you mention (dividing by 6.5) makes a judgement that I don't think needs to be made. Furthermore, the above version of the formula produces a number with an easily understood physical interpretation. Spread index, as defined above, is the diameter the diamond would have if it were rescaled to weigh exactly one carat. Regarding using spread ndex to gauge cut, I wouldn't. It is only a very crude indication.

As far as your diamond goes, a "perfectly cut" 0.84ct diamond should have a diameter equal to approximately 6.133mm. In addition, a "perfectly cut" diamond having the same average diameter as your diamond would weigh about 0.796ct.

Is this something to lose sleep over? I doubt it. For comparison, I ran the numbers on all Hearts on Fire diamonds within two points of yours. The average spread index of these 19 diamonds was 6.473 with the lowest spread index being about the same as the diamond you bought. It sounds like you did okay to me. Enjoy your new diamond.

 

[drdeano]

Thank you for the replies!

Like I said, I am probably being too picky...but I pride myself in being as educated as possible when it comes to major purchases. I was relieved that there were some numbers involved in diamond buying, rather than having to trust my eye..which has never really taken the time to look at diamonds (I feel uncomfortable looking at other people's jewelry for some reason.)

I trust your opinions after all the reading I have done. There certainly is ALOT to know! I appreciate the input.

-DrD