Dissecting the minor facets and exploring their impact on light return & brilliancy

Out of 57 (or 58 if there is a culet) facets a whopping 40 of them make up what we know to be the minor facets.  These include the star facets (8 total, colored in blue), upper girdles (16 total colored in yellow) & lower girdles (16 total, colored in red).   These are going to be the facets we'll primarily be discussing in this tutorial.

Most sophisticated measuring devices such as the Sarin & OGI MegaScope (we have both here) only give averages or facet by facet analysis of what are considered the major facets which consist of 8 kite or bezel crown angles/height and 8 pavilion main angles/depth plus the table size, girdle measurements and culet size. These account for only 17-18 of all the facets on the diamond.

Here is a top down analysis of these major facets on the OGI.  All measurements all color coded and the 8 kite/bezel facets and 8 pavilion mains (the facets that are measured to determine crown/pavilion angles/height are colored in yellow.  The table is colored in red.

While the measurements of the major facets will to an extent tell us a lot about the diamond in question it DOES NOT dictate the "look" or total optics of the stone and changes in the minor facets can affect how the diamond looks to the human eye and the diamonds overall brilliance.  When showing clients diamonds with similar measurements in major facets but noticeable differences in the minor facets MANY people can see and observe these differences without the aid of scopes/measuring devices.

Upper girdle facets

The upper girdle facets are those that go around the perimeter of the diamond which meet the star facets at their "point".  In the below graphic the upper girdles are indicated in the yellow.

The angles to which the upper girdle facets are cut can either increase or decrease light leakage around the periphery of the stone having a number of effects on the appearance of the diamond at this point.

Update 2006: Interestingly, when I initially wrote this article back around 2003, I didn't fully realize at the time how important the role of the cutting of the (upper) girdle facets would play in the newly released GIA and AGS Cut Grading systems.  Below are my unaltered observations and original publishing before any knowledge of the new cut grading systems.  It is amazing how what I had written back then corellates with the most conservative grading now carried out in each of the labs that are inspecting these features covered in this article.  Further updated references will be made below when necessary.  While we encourage the reading of this article which covers the upper girdle facets, star facets and lower girdle facets, we would also refer you to our most recent article on these girdle facets and how they impact a diamonds cut grade in the new cut grading systems starting here which you'll want to read after this.

Example #1 .60ct I SI1 H&A. 

Upper girdle angles ranging from 45°~47.4° or average 45.81° with average lower girdle facets at 41.6°.

It is my consistent finding in diamonds with upper girdle angles over 45° that it reduces both light return and contrast within the diamond and has the most negative impact upon the diamond in terms of it's cut quality.  The real world observation to those who look closely at these are grayed out edges which can reduce the "effective diameter" of the diamond.  Since this portion of the stone is not effectively returning light or contrast to the eye of the observer they are the upper girdle angles that we'd seek to avoid in the critical analysis.  Granted there are other more important aspects to reflect upon when it comes to minor facet analysis but if we have a choice ... we want to review more carefully the next 2 alternatives.

Note the white, pale white and weak redish areas in this example between the arrow tips. This amount of weak light return impacts the stone and reduces to an extent it's *effective diameter*.  The arrows will typically account for strong fire within the diamond but the weak light return around the perimeter (to the left of the green line) causes the diamond to take a hit in other areas.

Here is an upper girdle report on a diamond similar to the one above.

 
Update 2006: With the release of the new GIA and AGS cut grading systems, stones as used in the above example are now what are referred to as "dug out" or "digging" of the girdle facets and do not make GIA Ex or AGS Ideal grade in the new systems.  In the above example it is the upper girdle facets that are dug out.  For a more in depth article on digging and new technologies that reveal this feature please read our article on "Dug Out Girdles".

Example #2 .80ct G SI1

Average upper girdle angles 41.88° with average lower girdle angles of 41.3°

Diamonds with upper girdle angles between 40-43 degrees have increased light return to the point we we'll observe strong dark reds under the LightScope.  This is one very positive aspect between these and the above example.  Another positive feature is that although there is a tiny bit of leakage at these points, that small amount of leakage actually provides great contrast brilliance in this portion of the diamond. Brilliance is not only measured by the amount of light return within the stone but also the "contrast" that is picked up by the human eye.

By reducing the upper girdle angles to around 42° instead of 46° we eliminate a lot of light leakage.  Strong dark reds now replace the pale whitish reds at the perimeter.  There is still some leakage but now it has been greatly reduced to the point where it's barely noticeable at all to the human eye.  Upper girdle facets cut to these angles emphasize the greatest contrast between the 3 examples given in this tutorial.

Both examples 1 and 2 have very similar lower girdle angles (41.3 and 41.6) the primary difference being the upper girdle angles. (41.88° & 45.81°)

Here is an upper girdle report on a diamond cut with very similar proportions.

Example #3. 1.09ct H SI1

Elimination of leakage around the girdle.

Average upper girdle angles = <40°  Average lower girdle angles = 41.3°

Reducing the upper girdle angles to under 40° (generally 36-39°) brings the upper girdle/lower girdle angle relationship to a point where total leakage is eliminated (no white in LightScope).  While this is a positive aspect there are 2 other things to consider in this equation.

1. Contrast brilliance is lost due to the "blending" of the reds (ie. less contrast) and

2. The light return taking place at this juncture is reduced or weaker light return as indicated by the pale reds.

Few super ideals on the market have upper girdles cut to these angles.

When attempting to generate upper girdle reports on diamonds whose upper girdle angles are very close to the actual crown angles, both the OGI and Sarin machines have difficulties making the separations between the facets.  Hence the face up graph many times looks similar to this (or messier). ;)

 
Update 2006: The above example in the new cut grading systems are referred to as "painting" or "painted girdle facets".  In my original publishing I note the decrease in light return and contrast and interestingly these types of stones DO NOT make GIA Ex grade either.  Since AGS is using reflector based technololgy in their new system, they are a bit more liberal with these types of stones and they tend to squeak by with an Ideal grade so we agree with GIA's more conservative approach to grading this feature. Our more in-depth article on "Painted Girdles" can be found at this link.

Lessons to be learned concerning upper girdles. 

1. Steeper angles around 44 degrees and over cause more leakage around the perimeter as observed in LightScope/FireScope/IdealScope analysis.

2. Angles around the 40-43 degree range increase contrast (on/off effect) and cause intense light return to be where leakage was in the example above.

3. Upper girdle angles less than 40 degrees increase light return as seen in reflectors (albeit weaker at those points), decrease contrast but positively influence "spread".  In FireScope/IdealScope analysis all reds appear at the same color and makes for a more pleasing image under those viewers however real world observation testing shows these to be less favorable than option 2. These upper girdle angles eliminate blatant leakage so there will be no *whites* in the LightScope/FireScope/IdealScope analysis.

4. Bottom line. Points 2 and 3 above have proven to be the most appealing to laymen aesthetically HOWEVER of all the minor facets the upper girdles have the LEAST impact on the look and beauty of the stone.  So while we do look at this feature it does not take precedence over the impact that the stars and more importantly the lower girdle facets have on the aesthetic beauty of the diamond.

Update 2006: Reading my conclusions back in 2003 and with the further research we've conducted on this subject I'd stand by the statements made at that time but one thing I would change is the lack of emphasis placed on girdle facet cutting I had made in point 4.  While it was a feature we always inspected, the release of the new GIA Cut Grading System has emphasized the importance of these facets and how they are cut. Currently diamonds that feature painting and digging are noted on GIA Reports wherein these features affect the cut grade and are noted as "brillianteering of the half facets".

The Star facets

The star facets surround the table of the diamond (colored here in blue) and there are 8 total.

The star facets are the windows to the hot spots (blacks observed in LightScope analysis) between the arrow heads which are directly influenced by the lower girdle facet length.  When diamonds have proper lower girdle facet length, opening these windows increases the *sparks* that fly out from between the fiery arrows, thus increasing both fire and scintillation as observed in direct light conditions.  Closing these windows decreases that optical effect. GIA's most recent research shows exactly what I've been discovering concerning this phenomena.  Here are examples of diamonds with varying star/upper girdle ratios.

Here is a chart with graphics featuring diamonds with different star lengths.

The thing to be learned from the above graphic is how the light return through the stars are directly affected by the lower girdle facet lengths. The 2nd and 4th stones have identical star facet lengths yet one has distinct dark blacks at the stars while in the other the blacks are non-existent. The thickness of the pavilion mains are also notably different in the 2 stones which is directly affected by lower girdle facet cutting as well.

Update 2006: We have videotaped these differences in appearance in our Video Tutorial "The Nature of Scintillation" at this link.

In a Brilliancescope analysis the effects of this can be seen usually (but not always) in LightView #1.

The most intense bursts of light are coming through the star facets which have a nice pronouncement of black as observed in the LightScope analysis.  As you can see, if we lengthen the lower girdles by just 2.6% (from 78 to 80.6%) increases the fire and scintillation at those points within the diamond.

Below are graphics we've used simulating LightScope results via the DiamCalc software from MSU.  The default proportions used in this example are for a 6mm round diamond (.78ct) with 34.5 crown angles, 40.8 pavilion angles and a 56% table. The only parameters being changed are the stars.

Lower girdle facets

After reviewing the above chart, NOW is a great time to introduce you to how important the impact is when we adjust lower girdle facet length.  It is my professional opinion that the cutting of the lower girdle facets can dramatically affect the appearance of the diamond just as much as the pavilion mains and bezels.  Adjusting the length/height of the pavilion mains by just 1 or 2% can really make or break the difference between a rare top of the line rock vs a "very good" stone.

Measuring the lower girdles:

There are currently 2 ways to measure the lower girdles which can get a little confusing.  One way is measuring the "length" of the lower girdle facet while the other way is measuring the "height" of the lower girdle facet.  Let me demonstrate.

Now that we have that in order I'll be using the "lower girdle facet height/depth measurement" since the MSU software uses that measurement in it's software.  (That is the measurement I've been using all along in this tutorial so far too).

Now ... if you can think back a moment to the chart I was using above where all the constants were the same except the stars.  Now all the constants will be the same except we're going to change ONLY the lower girdle facet measurements. The constants being a 34.5 crown angle, 40.8 pavilion angle, 56 table and 55% stars.  You'll see why I think this (the lower girdle facets) is another VERY important feature of the diamonds cut quality.

Now that we've covered all that we're proud to introduce you to a Sarin analysis that is exclusive to the UDIS.  You can now see via our Sarin DiaVision Reports the details concerning

• Minium, maximum and average upper girdles angles!

• Minimum, maximum and average star/upper girdle length!

• Minimum, maximum and average lower girdle length!

This is another UDIS first and we're proud to bring it to you. To view the minor facet details simply right click on this button

and choose "Save Target As" in the menu and place it in your folder...

C:Program FilesSarin TechnologiesWeb ViewerSrnViews(2)

Then when you open a Sarin DiaVision file, represented by this button,

you'll have an option to view a Report labeled "GoodOldGold" and in that report you will see everything from soup to nuts concerning the diamond and it's measurements.

Information to note in the GoodOldGold Report are the

• Average upper girdle angles highlighted in the blue rectangle.

• Average lower girdle angles highlighted in the green rectangle.

• Average lower girdle facet length highlighted in the red rectangle.

• Star/Upper girdle ratio highlighted in the yellow rectangle.

  • Further information to note are the variances (min/max) of the upper girdle angles, upper/star ratio, & lower girdle angles/length.

A gemological appraiser can gain much from this information once they've grasped the understanding of the optical effects the minor facets have upon the appearance of the diamond.  If you'd like to read a review of some of GIA's latest research on the minor facets and how it corresponds to our own here is a link to an interesting article. http://www.professionaljeweler.com/archives/articles/2002/jan02/0102dg.html

FAQ's

Question: Is it possible to tell the optical results on the minor facets by reading a general Sarin or OGI Report?

While both Sarin & OGI each make instruments to measure the minor facets, most software/hardware packages that stores/labs own do not show this data.  At best you may find a report giving star/upper girdle ratio or just star facet length.  If the dimensions and minor facet measurements are know it is easier to assess optical properties of the entire stone rather than attempt to make a prediction based on only the results of 18/58. So the short answer to your question is no.

This diamond below is a good example to learn from.

Not only do all it's proportions fall in the AGS "0" zone but it's variances are super tight (min/max's).  One would automatically assume a *perfect* Hearts & Arrows optical symmetry but when we look at the photographs we see a different story.

Haha! Ok.. so I'm being anal.  Yes we do have here what many manufacturers & stores would consider a "true" Hearts & Arrows diamond (and perhaps I am being a tad crude in my analysis, the stone really is gorgeous and we did purchase it but...). A variance that was too wild in one of the lower girdle facets caused one heart, in the 8:00 position, to be smaller than the rest, thus disqualifying it as a "true" H&A. Smaller hearts are the result of lengthening lower girdle facet length.  And on the contrary shortening lower girdle facet length increases the size of the hearts.

We can see the direct results of this in the face up view between the 4:00 & 6:00 arrows in the "arrows pic" under the table facet and also in the LightScope image between the 7:30 & 9:00 arrow shafts under the table.  Note that increasing lower girdle facet length increases the blacks in the LightScope image at that point (thus increasing DCLR).