Girdle, Polish and Symmetry

Girdle

For Grading The Thickness Of The Gem/Mounting Interface And The Center Held Carat Weight

The girdle is the outside edge of the diamond, the circumference edge. Four main reasons for grading girdle thickness are (1) the quality of the gem/metal interface, (2) the girdles visibility, (3) variance in the girdle thickness and (4) the possibility of excess carat weight contained in the center, most weight intensive area of the diamond.

Gemological Institute of America describes eight girdle sizes. The percentages below are the contribution of the excellent American Gem Society diamond grading system.

Extremely Thin Girdles, sometimes described as a knife–edge, display a sharp edge under ten power magnification. This girdle is subject to fracture at the gem/metal interface in jewelry setting and in active wear when the diamond is subjected to impact.

1) This is a GIA Make Class Four grade for all diamond shapes.
2) An AGS grade 7 measuring 0% to .09% of the girdle diameter.
3) An AGA below average grade for fancy shape diamonds.

Very Thin Girdles show a very thin line at ten powers that is barely visible to the naked eye.

1) This is a GIA Make Class Three grade for all diamond shapes.
2) An AGS grade 1 described as measuring .091% to 0.50 % of the of the girdle diameter.
3) An AGA average grade for fancy shape diamonds.

Thin Girdles show a thin line at ten powers and are difficult to see with the naked eye.

1) This is a GIA Make Class Two grade for all diamond shapes.
2) An AGS 0 (zero) grade for Ideal cut diamonds measuring between 0.51% and 1.20% of the girdle diameter.
3) An AGA fine grade for fancy shape diamonds.

Medium Girdles show a distinct line at ten powers and a thin line to the naked eye.

1) This is a GIA Make Class One grade for all diamond shapes.
2) An AGS 0 grade for Ideal cut diamonds measuring between 1.21% and 1.70% of the girdle diameter.
3) An AGA fine grade for fancy shape diamonds.

Slightly Thick Girdles are obvious under ten powers and distinct to the naked eye.

1) This is a GIA Make Class One grade for all diamond shapes.
2) An AGS 0 grade for Ideal cut diamonds measuring 1.71% to 2.95% of the girdle diameter.
3) An AGA fine grade for fancy shape diamonds.

Thick Girdles are very obvious at ten powers and obvious to the unaided eye. Because oil and dirt can collect in the coarse, bruited (unfinished) surface, it is considered good technique to facet (polished individual surfaces all around) the girdle.

1) This is a GIA Make Class Two grade for all diamond shapes.
2) An AGS 3 grade measuring 2.96% to 4.20% of the girdle diameter.
3) An AGA fine grade B for fancy shape diamonds.

Very Thick Girdles are distracting at ten powers and very obvious to the naked eye.

1) This is a GIA Male Class Three grade for all diamond shapes.
2) An AGS 5 grade measuring 4.21% to 5.70% of the girdle diameter.
3) An AGA average grade A for fancy shape diamonds.

Extremely Thick Girdles are distracting to the naked eye.

1) This is a GIA Make Class Four grade for all diamond shapes.
2) An AGS 7 grade measuring 5.71% to 6.20%. AGS further classifies thickness to grade 10, measuring in excess of 7.21%.
3) An AGA below average grade for fancy shape diamonds.

Girdles will vary in thickness from one part to another part of the girdle. In these cases, grades are set by the thickness of the most extreme measurement.

Because the girdle is at the most wide part of the diamond the girdle area is the most carat intensive portion of the diamond. Thick to extremely thick girdles can hold increased percentages of the overall carat weight causing a reduction in the girdle diameter and with extremely thick girdles causing as much as a 25% reduction in the diameter measurement due to excessive carat weight retention in the center of the diamond.

Polish and Symmetry

For Grading The Finish Of The Exterior Facet Surfaces And The Alignment Of The Diamond, One Part To Another:

The most understandable grading system for diamond finish, the polish quality of the exterior and the alignment of one part of the diamond to another, is provided by Gemological Institute of America’s Make Class system.

GIA expresses polish of the facet surfaces in five categories:

– Excellent polish means the facet surfaces or facet junctions (where one facet meets another) would have very few and hard to find polish lines and/or blemishes.

– Very Good polish means the facet surfaces or facet junctions would have only faint polish lines or insignificant blemishes in inconspicuous places.

– Good polish means a few transparent polish lines are found on the crown (top) of the diamond and/or when there are several small blemishes.

– Fair polish means the diamond shows obvious polish lines or marks on several facets or has noticeable blemishes

– Poor polish is graded because of polish marks and/or blemishes that reduce transparency.

The human eye cannot differentiate between the GIA Excellent and Good polish grades. Although under a microscope view of a diamond with Excellent polish it's clear the cutter has spent valuable extra time in finishing the diamond and that such effort is not justified for other than the highest value VVS and better, F and better, Make Class One or Ideal cut diamonds. For all other diamonds, unless the polish grade falls at Fair or below, polish is not a primary determining factor of naked eye visible brilliance or beauty.

The Gemological Institute of America Make Classes for polish grading are as follows:

– Make Class One: Polish graded Very Good to Excellent
– Make Class Two: Polish graded Good
– Make Class Three: Polish graded Fair
– Make Class Four: Polish graded Poor

Symmetry addresses the alignment of one part of the diamond to other parts of the diamond.

There is an ideal image of perfection for each diamond shape whereby the table is perfectly octagonal and centered, the girdle is perfectly shaped, the crown facets are all perfectly placed and appropriately shaped and in alignment with the pavilion facets which are, likewise, perfectly placed and appropriately shaped, with the culet perfectly centered and where the plane of the girdle is perfectly parallel to the plane of the table. Only the highest color (colorless) and clarity (flawless to near flawless) diamonds are cut to such exacting standards because at that nature given level of perfection there is a demand for perfection in cut to maintain the pedigree of such diamonds. Most diamonds are shy of that perfect ideal because the time and effort required of perfect cutting does not result in extra dollars to the cutter. The naked eye cannot readily distinguish between an Excellent cut and a Good cut.

The symmetry characteristics that are taken into consideration in determining a GIA Make Class grade include:

– The table and/or the culet may be off center.

– The girdle (circumference) of the diamond may be out of round.

– Facet surfaces may fail to point correctly.

– There may be a misalignment of the crown and pavilion facets whereby the facet junctions do not align.

– The table (top center facet) may not be a true octagon.

– Facets may be out of normal shape.

– The plane of the table and the girdle may not be parallel.

– The girdle may be wavy.

– There may be extra facets (a technique used to eliminate blemishes or inclusions)

– There may be present natural (raw) crystal surfaces that do not effect the clarity grade.