OGI Mega Scope Computerized Proportions Analysis

Would you purchase a new car without knowing whether it had a four, six or eight cylinder engine? Of course not! Then why would you consider buying a diamond without knowing how well it has been cut? After all, most of the visual properties of a diamond are controlled by the Crown and Pavilion angles...

Assuming that a diamond has been cut to "ideal proportions" simply because it has the "right" table and total depth measurements is foolish, but unfortunately this is what many consumers are being asked to do by disreputable dealers who rely on diamond grading reports that fail to provide complete information.

In order to determine the proportions rating of a diamond, you must know the specific measurements for the following factors:

Table Diameter
Crown Angle
Pavilion Angle
Girdle Thickness
Culet Size

However, most of the diamond grading reports relied on by diamond buying consumers only contain the following measurements:

Table Diameter
Total Depth
Girdle Thickness
Culet Size

Thus the average measurements for the Crown & Pavilion angles are undetermined and there is no way of actually knowing the proportions rating for the diamond... The importance of knowing the measurements for these two factors is that together they control about 98% of the visual properties of a diamond...

The easiest and most accurate way to determine the specifics of the missing factors (Crown & Pavilion angle) is to obtain an OGI or Sarin computerized proportions analysis. Both the Sarin and the OGI machines offer the same basic information and level of accuracy, the choice to use one machine over the other is really a matter of personal preference, similar to how you may choose to drink Coke instead of Pepsi.

In the past, we relied on a Sarin machine, but switched to an OGI Mega Scope due to service related issues that were left unresolved by Sarin. The unit we use in our office cost a little more than $45K and provides us with better optics and a higher level of accuracy that is not available from the smaller, less expensive units used by many diamond retailers. In addition, the larger machine enables us to determine the proportions rating for round brilliant cut diamonds up to thirteen carats and a larger range of fancy shape diamonds.

In the past, the results of the Sarin and OGI computerized proportions analysis were printed on an Avery type label that looked like this:

However significantly more detailed reports are currently available as will be discussed in detail below... We will explain how to decipher the information provided on the label however because we attach this label to the scan of the diamond grading reports we post on-line in addition to making the full version of the proportions analysis available...

The top line on the label format provides a reference to the lab report of the diamond that was measured followed by the AGS proportions rating for the diamond which in this case is "AGS 0" followed by the carat weight of the stone as estimated mathematically by the program based upon the measurements obtained by the machine. It is important to note that the reference to the AGS proportions rating does not mean that the AGS Laboratory (AGSL) actually graded the diamond has having AGS Ideal 0 proportions, but rather that the machine was equipped with the software parameters to compare the proportions of the diamond to the scale created by the AGSL. Thus the initials "AGS" which appear after the lab report number do not indicate that the proportions were determined by the AGS Laboratory, but rather that we used the AGS proportions option on the OGI machine which compare the measured proportions of the diamond to the parameters set forth by the AGS Laboratory to determine the proportions rating. In addition, we would like to state that the OGI and Sarin machines are not capable of grading the polish, symmetry, clarity or color of diamonds, the machines only grade the proportions...

In this case, the second line which begins with "diam" indicates that the average outside diameter of the diamond is 6.56 mm which is the average of the minimum (6.54 mm) and maximum (6.57 mm) measurements and then states the depth measurement which in this case is 3.97 mm.

The third line "crown" provides the average crown angle measurement (34.5 degrees) which is the average crown angle measurement based upon the eight individual measurements of the kite shape bezel main facets. The percentage measurement (14.7%) is not actually measured by the Sarin or OGI machines, but rather is estimated mathematically, thus the angle measurements determined by the machine are more accurate because they were actually measured.

The fourth line "pavil" provides the average pavilion angle measurement (40.8 degrees) which is the average pavilion angle measurement based upon the eight individual measurements of the triangular lower girdle facets. The percentage measurement (43.0%) is not actually measured by the Sarin or OGI machines, but rather is estimated mathematically, thus the angle measurements determined by the machine are more accurate because they were actually measured.

The "table" measurement is provided in two formats, millimeter diameter and a percentage of the total diameter, the millimeter diameter can be misleading so focus on the percentage... Here again, the table diameter % is based upon an average of the measurements.

Culet % refers to the size of the culet which is facet that makes up the bottom point of the diamond. We actually ignore this measurement on the Sarin and OGI results because they seem to vary greatly and seem to be the one factor for which the results never make sense... Neither Sarin or OGI have been able to provide us with what we consider to be a reasonable explanation for this apparent phenomena, so we just go by what the labs determined the culet size to be...

The "girdle" measurement provides the average girdle thickness of the diamond, depending on the format selected when the diamond is measured the average can be based on the high points or valleys of the girdle and thus here again, we prefer to rely on the measurements provided by the lab that graded the diamond for consistency.

When the Sarin and OGI machines were first introduced on the market, the label which provided this information was "state-of-the-art" and thus was an excellent resource for consumers. However recent technological advances and improvements to the software packages used by the Sarin and OGI machines have made it possible to provide consumers with more information than used to be available. This is what the new OGI MegaScope Reports look like:

The primary difference between this report and the results provided on the label shown previously on this page is the amount of information that is made available to the consumer. With such detailed information now being available, we don't know why anybody would accept the basic label format anymore... Not only are the averages shown on the new report format, but the high and low measurements that resulted in those averages are also provided in the upper left section of the report pictured here:

In this case, the average of 34.5 degrees for the crown angle is based on a low of 34.3 degrees and a high of 34.7 degrees, but it could just as well have been based on a low of 34.0 degrees and a high of 41.0 degrees... Obviously, the tighter the range between the high and low measurements the better...

The proportions chart located right beneath this section provides the parameters for the proportions scale developed by the AGS Laboratory:

Please note however that the chart above uses the pavilion depth percent as a determining factor of the proportions rating and that during the first quarter of 2003 the AGS eliminated the percent variables from their proportions scale because of a realization that the angle measurements are more accurate than the estimated percentages... The range for the pavilion angle measurement for the AGS Ideal 0 proportions grade ranges from 40.15 to 41.20 degrees. While both Sarin and OGI have been made aware of the change of parameters for the AGS zero ideal cut rating, the program software has not been updated yet to reflect the change and thus some diamonds that would score "zero" on the current AGS proportions scale are reflecting lower scores...

The "Average Graph" which appears in the upper right corner of the report is simply a side profile of the diamond based upon the proportions of the diamond. Starting at the top, the 6.56 mm indicates the average outside diameter for this particular diamond. The 56% listed just below that represents the table diameter and the 3.67 mm indicates the millimeter diameter of the table. The crown angle of 34.5 degrees is indicated in red along the left edge of the crown and the pavilion angle of 40.8 degrees is indicated in red along the left edge of the pavilion. The percent measurements for the crown (14.7%) and the pavilion (43.0%) are indicated along the right side of the stone, but be aware that the angle measurements marked in red are more accurate because those measurements are actually measured by the machine and the percent measurements are estimated... The total depth of 60.6% is indicated in the lower left corner of the graph and the culet size is indicated at the bottom.

The Crown & Pavilion Graph merely indicates the ratio of alignment between the center of the table facet to the center of the culet located at the bottom of the stone. Think of this as simply drawing a line through the stone from the center of the table facet to the center of the culet... The reality is that we have never seen a diamond that didn't have a culet that was indicated as being "off center" but we prefer to see the measurement below 0.3%

The "Girdle Profile" is exactly what it seems to be, it is a representation of the consistency of how well the girdle edge of the diamond has been formed. The minimum and maximum girdle measurements are indicated along with a graph showing the pattern of the girdle edge. In this case, the girdle has been cut very nicely and does not appear to be excessively wavy or erratic in pattern. The following is a picture of a girdle edge which is not acceptable and should be avoided:

The "Top Down Report" (pictured below) provides excellent insight into the consistency of the cut, but can be a source of confusion for consumers who do not take a moment to understand how to correctly decipher the information provided.

To begin deciphering the information provided in this report we want to explain the numbers indicated on the graph, starting at the top...

The green number "7" indicated in the twelve o'clock position merely identifies the facet number, the kite shape facet in the twelve o'clock position is actually the 7th Bezel Main facet...

The red numbers "34.3" that appear below that represent the Crown Angle measurement for the 7th Bezel Main facet...

The blue numbers "14.4" that appear below the Crown Angle measurement represent the Crown Height Percent for the 7th Bezel Main facet, and so on... As stated previously, the angle measurements are more accurate than the percent measurements because the OGI and Sarin machines actually measure the Crown and Pavilion angles but estimate the percent readings...

The most common mistake consumers make is to attempt to decipher the information provided on the Top Down report by comparing the measurements provided clockwise and judge the consistency of the diamonds' cut based on that point of view. Thus using the example pictured above, they would incorrectly judge the consistency of the diamonds' cut by comparing the Crown Angle measurement of Bezel Main facet 7 to the measurement for Bezel Main facet 8 to that of Bezel Main facet 1 and so on...

The correct way to decipher the information however is actually to add the measurements of each opposing facet together and evaluate the consistency based on the sum of those measurements. In other words, you should not determine how well this diamond has been cut by comparing the Crown Angle measurements starting at the twelve o'clock position and reading around in a clockwise circle, but rather by the sum of the averages as follows:

Facet 7 (34.3 degrees) plus Facet 3 (34.5 degrees) = 68.8
Facet 8 (34.5 degrees) plus Facet 4 (34.3 degrees) = 68.8
Facet 1 (34.7 degrees) plus Facet 5 (34.4 degrees) = 69.1
Facet 2 (34.5 degrees) plus Facet 6 (34.4 degrees) = 68.9

Thus the average consistency for the Crown Angle is within three one hundredth's of a degree... Obviously the tighter the range, the better the consistency of cut... Deciphering the information for the Pavilion Angle is done the same way, based upon the sum of the opposing facets... The pavilion angle measurements are indicated in red with the percent measurements indicated in blue... However, the location of Facet 7 appears at the bottom of the graph because you are looking at the bottom view of the stone, thus if you were to rotate the position of the bottom view graph so that it faced bottom the two halves would align top to bottom...

There are many other report formats available for each diamond listed on our Private Reserve although we do not generally publish that information because most of it consists of single page reports which indicate the same information provided on the full page report... For instance, there is a full page version of the Crown & Pavilion Graph pictured above which simply consists of a larger version of the graphic and does not provide any additional information... If you happen to be a glutton for punishment and feel the need to sift through the individual pages which are condensed into the information provided on the full page report, we'll be happy to send them to you upon request...

AGS® is a registered trademark of the American Gem Society (702) 255-6500
GIA® is a registered trademark of the Gemological Institute of America (760) 603-4000

OGI MegaScope Computerized Proportions Analysis: