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Comparing Diamond Certificates

Comparing Certificates:
All Is Not Equal

Understanding Diamond Certificates

It is a known fact that a diamond's make affects the stone's ability to return light, therefore a badly proportioned diamond will not be as "bright" or "fiery" as an Ideal cut or well proportioned stone.

It is then of course logical, that an ideally cut diamond should command a higher sale price. And as the quality of the workmanship on a stone declines and its proportions move away from a given standard of excellence, the stone's value should decline.

Each Certification house has its own set of parameters (yes, sadly it's true. There is NO standard). Some don't even have proportion standards and those who do, may have proportion parameters that are so wide that almost every stone graded is given a favorable grade. Obviously this will make a customer believe that they have purchased a stone of premium quality.

Is that fair? Sure, if youre a jeweller and you are still under the illusion that "Buyer Beware" applies, then good luck to you. Thankfully, many believe those days are quickly passing. We are dealing with a new, educated client seeking premium goods, requiring assurance and guarantees. Unfortunately, sometimes our word is just not good enough.

How strange!

It has been told to us on good authority that the reason the HRD certificate does not have an Ideal or Excellent grade is that they do not consider it their job to tell the customers what is good or bad. So why do they even have a proportion grading scale? The top grade they use is Very Good…. but with proportions so wide almost every stone will pass as Very Good.

It wasn't until recently that the GIA has had to include a cut grade. Pressure from the industry demanded it. Originally the GIA certificate showed the stone's proportions, but did not give a grade for them. They did grade and report on the stone's polish and symmetry. This was a little odd, as you can have an excellent polish and symmetry on a stone that has a 70% table and 53% depth! Now is that an Excellent? Even though the GIA certificate now includes a Cut grade they do not publish their proportions thus it is almost impossible to gauge exactly what is going on.

It is also noted that some certificates in the marketplace grade to what is called I.D.C. ruling. (This is basically the HRD grading system.) Check the certificate carefully as certain independent I.D.C. labs have introduced an "Excellent Cut" grade contrary to the I.D.C. rules. The "Excellent" grade proportions aren't available to members of the trade or public alike.

Now, there are other certificates apart than HRD and GIA. Some are built on integrity and honesty, and some aren't worth the paper they are written on. They all have their own proportion parameter fields (just as Auscert does) but it is vital for you to understand the calculations behind them. Many certificates carry the term Ideal and Excellent grade but just what goes into these Makes?

Many certificates have wide proportion parameters, thus a disproportionate number of average stones will fall into their top grades. Can you believe that there is a commercially available certificate that now has an IDEAL PLUS grade!

Apples & Apples – We don't think so!

Now you understand the different areas that are used to grade a diamond's Make, below are some of the most commercially used and well-known certificates' parameters fields.


Sept 08 - IDC Has announced an introduction of an Excellent cut grade from January 2009. No parameters have been released to date.

 Very GoodGoodUnusual
Table %53 – 6651 – 67>51 – <67
Total Depth %55.5 – 63.953.5 – 66.9>53.5 – <66.9
Pav Depth %41.5 - 45.040 – 46.5>40 – <46.5
Pav Angle39.6 – 42.238.5 – 43.1>38.5 - <43.1
Crown Angle30.7 – 37.727 – 40.6>27 – <40.6
Crown Height11 – 169 – 18>9 – <18
Girdle2 – 4.51 – 7.5>1 – <7.5
CuletN – 1.9 %2 – 3.9Over 4%


 IdealExcellentVery Good
Table %53 - 6052.5 – 63.551.4 – 63.5
Total Depth %58-5 – 62.556 – 6353.5 – 63
Pav Depth %42 – 4441.8 – 44.341.5 – 44.8
Pav Angle   
Crown Angle33 – 3632.7 – 36.331.7 - 36.8
Crown Height   
GirdleThin – Sl.ThickThin – Sl.ThickThin – V.Thick


 Ideal PlusIdealVery GoodGoodFair
Table %53 – 5858.1 - 6363.1– 6565.1 -6767.1 – 71.5
Total Depth %58.5 – 62.557 - 63.556 – 64.554 – 66.950 – 69.9
Pav Depth %42 - 4441.5 – 44.541 – 44.640 – 4637.9 – 46.1
Pav Angle     
Crown Angle33 – 3632 – 36.331 – 36.830 – 37.829.5 – 38.8
Crown Height     
GirdleThin – Sl.TkThin – Sl.TkTh – TkV.Th – V.TkEx.Th - Ex.Tk
CuletNil - medN – Sl L.N – Sl. LN - LN - VL


Table %N/AN/AN/A
Total Depth %N/AN/AN/A
Pav Depth %N/AN/AN/A
Pav AngleN/AN/AN/A
Crown AngleN/AN/AN/A
Crown HeightN/AN/AN/A


  0 1 2 3 4
Table % 52.4-57.5 51.4-59.5




Pav Angle 40.2-41.2 40.2-41.5 39.8-41.8 39.8-41.8 39.5-42.1
Crown Angle 33.7-35.8 32.7-36.3 32.2-36.8 31.7-37.3 31.2-37.8

AVG: centre slightly thick

MIN: Centre slightly thick

MAX: Centre slightly thick

AVG: Centre slightly thick

MIN: Very thin-slightly thick

MAX: Very thin-slightly thick

AVG: Centre slightly thick

MIN: Very thin-slightly thick

MAX: Very thin-slightly thick

AVG: Thin

MIN: Extra thin- thick

MAX: Extra thin- thick

AVG: Very Thin

MIN: Extra thin- thick

MAX: Extra thin- thick

Culet Pointed-medium Pointed-slightly large Pointed-slightly largePointed- large Pointed- large


As from the January the 1st 2006 the G.I.A. started producing certificates, which finally included a cut grade. Several trade publications have been issued introducing the cut system but to our knowledge no definitive proportion table has been made available to the trade or the public, nor do the proportions table appear on the certificates - making it hard to establish exactly how they determine an Excellent make.

From our finding it would look as though they have split the proportions into five categories starting with Excellent, that allows the table measurement to go up to 61%. In the second category the table can go up to 63% with the fourth going up to 70%. As we discover any further information we will post it. (Anthony Bates 6th April 2006)

So how do we measure up?

At Auscert we only measure the table size, crown angle, pavilion angle, pavilion depth, culet and girdle. We do not consider the overall depth, for a stone that has a 60% total depth could be made up of 50% pavilion and 10% crown with no girdle. This doesn't make a nice stone but it does have 60% depth.

We believe that if the table size is correct with the tolerances of the crown and pavilion angles, the rest of the stone must mathematically fall into place. We add extra checks of measuring the girdle and culet. Finally the pavilion's depth is taken into consideration to make sure the stone isn't too "bottom heavy".


 IdealExcellentVery GoodGood
Table %53 – 57.552.4 - 60.552.4 – 62.450.5 – 64.8
Total Depth %    
Pav Depth %40.2 – 43.542.3 – 43.841.7– 44.741.1 – 45.5
Pav Angle40.2 – 41.040.2 – 41.239.8 – 41.839.4 – 42.3
Crown Angle33.8 – 35.332.7 – 35.832.2 – 36.431.2 – 37.8
Crown Height    
GirdleTH – Sl.TkTh – Sl.TkTh - TkTh – V.Tk
CuletPointed - MedP – Sl.Lp – Sl.LP – L

Any stone that fails to reach the GOOD grade will be refused certification.


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