Assignment 12/13 - Grading Proportions Flashcards
The result achieved by adding the smallest and largest diameter measurement of a round brilliant and dividing by two.
Average girdle diameter.
Table-to-culet depth, expressed as percentage of the average girdle diameter
Total depth percentage
How does average girdle diameter relate to other proportions?
Average girdle diameter is the basis for comparing a diamond’s other proportions.
How do you determine total depth percentage?
divide the total depth by the average girdle diameter.
How can you recognize overweight diamonds?
Generally a total depth percentage of >65%.
A diamond’s table size expressed as a percentage of its average girdle diameter
Table percentage
How do variations in a diamond’s table size affect its appearance?
A too large or too small table have a negative impact on brightness and fire. Most brilliants have a table percentage between 55% and 65%.
What are the methods for determining table size
- Direct Table Measurement (Table gauge & loupe)
- Estimating (Flash, Ratio or Bowing method)
A transparent measuring device divided into millimetres and tenth or a millimetre, used to measure a diamond’s table
Table gauge
The length of the star facets expressed as a percentage of the total distance between the girdle and the edge of the table facet.
Star length percentage
How do star facets affect a diamond’s appearance
They mainly affect a diamonds symmetry.
The angle formed by the bezel facets and the girdle plane.
Crown Angle
The distance from the girdle plane to the table, expressed as a percentage of the average girdle parameter.
Crown Height Percentage
What are the effects of a shallow and steep crown angle?
The have a negative effect on brightness and fire.
What techniques can you use to estimate crown angles?
- Profile Method
- Faceup Method
How does the girdle thickness affect a diamond’s durability and appearance ?
A girdle should be thick enough to prevent chipping but not so thick that it detracts from the stone’s appearance.
How doe you estimate girdle thickness?
Assess at 10x:
Extremely thing (ETN) - knife-edge
Very Thin (VTN) - very thin line
Thin (THN) - a thin line
Medium (MED) - distinct line
Slightly thick (STK) - obvious
Thick (THK) very obvious
Very thick (VTK): distracting
Extremly Thick (ETK): very distracting
A weight-retention method that adjust the angles of some facets to allow portions of a diamonds girdle to be thicker.
Painting
A weight-retention method where the cutter leaves more girdle thickness to allow for later removal of clarity characteristics around the girdle
Digging out
Where is the focus when describing girdle thickness?
Valleys
What facets are used to estimate the crown angle in profile?
Table and bezel facets
The quality of the polish and precision of the cut of fashioned gemstones
Finish
How do variations in pavilion dimensions affect a diamond’s appearance?
The pavilion makes a significant contribution to brightness and fire.
How are pavilion depth percentage and pavilion angle related?
The greater the pavilion depth percentage, the steeper (greater) the pavilion angle.
The distance from the girdle plane to the culet as a percentage of average girdle diameter.
Pavilion depth percentage
The angle formed by the pavilion mains and the girdle plane.
Pavilion angle
How does the girdle thickness percentage affect a diamond’s potential cut grade?
Girdle thickness can limit the cut grade.
>10% –> Lower cut grade
>= 11.5% –> Poor
How do you estimate the length of a diamond’s lower half facets?
Estimate face up under 10x magnification and calculcate the average.
What is the purpose of a diamond’s culet
The purpose of the culet is to prevent abrasion and chipping on loose diamonds.
The length of the lower half facets express ad the percentage of the total distance between the girdle and the culet.
Lower half length percentage
The size of the facet at the bottom of the diamond where the pavilion mains meet
Culet size
How do design and craftsmanship influence a diamond’s cut grade?
Both are included in the cut grade.
What effect does excess weight have on a diamond’s cut grade?
A diamond is overweight if it is heavier than expected by his (diameter) size.
A diamond’s physical shape, including its proportions and durability, determined by the decisions made during the fashioning process.
Design
The care that goes into the fashioning of a polished diamond, as confirmed by its finish
Craftsmanship
What are the causes of some common polish characteristics?
They can be the result of the finishing process or by blemishes created after cutting.
What are the quality ratings for polish and symmetry?
Both are rated seperately:
- Excellent (Ex)
- Very good (VG)
- Good (G)
- Fair (F)
- Poor (P)
What factors determine a diamond’s overall cut grade?
Table percentage, crown angle and pavilion angle play the biggest role in determining a round billiant’s brightness. Its important place emphasis on how the diamond actually looks rather than on its numerical parameters.
How do parameters at the extremes in their range impact cut grade?
The limit the maximum grade
How many crown angles on a round brilliant diamond should be estimated before averaging?
8
What may result from a thick, unpolished girdle?
Grey reflections
Why do diamonds with steep crowns offer less value?
They appear smaller than their weight suggests.
