Minor Facets, Major differences

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.

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.

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). ;)

Lessons to be learned concerning upper girdles.

Steeper angles around 44 degrees and over cause more leakage around the perimeter as observed in LightScope/FireScope/IdealScope analysis.
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.
Upper girdle angles less than 40 degrees increase light return (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. These upper girdle angles eliminate blatant leakage so there will be no *whites* in the LightScope/FireScope/IdealScope analysis.
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.

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.

46% stars	54% stars	60% stars	54% stars

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.

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

LightScope Analysis

LightView #1 with diamond

Same view with diamond stripped away.

Lower girdle report

Star facet report

65% stars

80.6% lower girdles

62% stars

78% lower girdles

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.

Star/Lower girdle height.	Simulated LightScope Image	Comments
43% stars / 80% lower girdles		Rich dark blacks can exist in short stars IF the lower girdles are cut to proper length.
55% stars / 80% lower girdles		Blacks under the stars are stretched slightly enhancing fire at those points.
65% stars / 80% lower girdles		Blacks are lengthened further.

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.

Lower girdle facet length measured via Sarin

Lower girdle facet depth/height measured by OGI

When measuring lower girdle facet length, the edge of the girdle to the culet is the 100% reference point as measured along the horizontal plane (indicated by the red arrow).

Lower girdles are measured on the horizontal plane (instead of the vertical plane) so when we examine the graphic from it's profile view we clearly see how this metric is determined (indicated by the green arrows).

Advanced hardware/software solutions from Sarin measure the lower girdles in this fashion and is generally 3% less than lower girdle facet depth/height.

When measuring lower girdle facet depth, the bottom of the girdle down to the culet is again the 100% reference point as measured along the vertical plane (again indicated by the red arrow).

The lower girdles are measured against the depth of the pavilion along the vertical plane (indicated by the green arrows).

Advanced hardware/software solutions from OGI measure the lower girdles in this fashion.

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.

Star/Lower girdle height/depth.	Simulated LightScope Image	Comments
55% stars / 73% lower girdles		Short lower girdles (73%) equals FAT ARROWS. Those wide pavilion main facets do provide for strong fire but reduced scintillation and oftentimes white light return. There are NO blacks at the stars and none at the arrow shafts. Less black = less intense light return from within as observed in direct light conditions. Here are some examples we've scanned in. http://www.goodoldgold.com/1_005ct_e_vs2_ags_0.htm http://www.goodoldgold.com/1_008ct_g_vs2_ags000.htm http://www.goodoldgold.com/1_06ct_f_vs1_ags0.htm
55% stars / 78% lower girdles		Increasing the lower girdle facet length to 78% produces better light return through the stars. The arrows are better proportioned within the diamond and a better sense of "balance" is acheived in the overall optical symmetry of the stone. In the example above (73% lgs) some feel there's a little too much black with regards to the pavilion mains (arrows) whereas the 78% lower girdles enter a zone that is more visually appealing. Note that leakage is eliminated at the girdle bezel area (where the arrows come to a point). Here isan excellent example of an ideal cut diamond with 78% lower girdle depth. http://www.goodoldgold.com/new_page_28.htm
55% stars / 81% lower girdles		81% lower girdles give more pronounced fire/scintillation through the stars and we can now see the presence of more blacks at the arrow shafts (see the green arrows). These diamonds produce some of the most aesthetically beautiful stones within the H&A diamonds and are usually TEAMING with fire and scintillation. Here is an example of one we've scanned in with 81% lower girdles. http://www.goodoldgold.com/1_03ct_h_vs1_h%26a1.htm
55% stars / 83% lower girdles

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 Files\Sarin Technologies\Web Viewer\SrnViews(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).