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GIA Graduate Gemologist - GEM 230 > Assignment 12/13 - Grading Proportions > Flashcards

Assignment 12/13 - Grading Proportions Flashcards

1
Q

The result achieved by adding the smallest and largest diameter measurement of a round brilliant and dividing by two.

A

Average girdle diameter.

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2
Q

Table-to-culet depth, expressed as percentage of the average girdle diameter

A

Total depth percentage

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3
Q

How does average girdle diameter relate to other proportions?

A

Average girdle diameter is the basis for comparing a diamond’s other proportions.

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4
Q

How do you determine total depth percentage?

A

divide the total depth by the average girdle diameter.

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5
Q

How can you recognize overweight diamonds?

A

Generally a total depth percentage of >65%.

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6
Q

A diamond’s table size expressed as a percentage of its average girdle diameter

A

Table percentage

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7
Q

How do variations in a diamond’s table size affect its appearance?

A

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%.

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8
Q

What are the methods for determining table size

A
  • Direct Table Measurement (Table gauge & loupe)
  • Estimating (Flash, Ratio or Bowing method)
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9
Q

A transparent measuring device divided into millimetres and tenth or a millimetre, used to measure a diamond’s table

A

Table gauge

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10
Q

The length of the star facets expressed as a percentage of the total distance between the girdle and the edge of the table facet.

A

Star length percentage

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11
Q

How do star facets affect a diamond’s appearance

A

They mainly affect a diamonds symmetry.

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12
Q

The angle formed by the bezel facets and the girdle plane.

A

Crown Angle

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13
Q

The distance from the girdle plane to the table, expressed as a percentage of the average girdle parameter.

A

Crown Height Percentage

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14
Q

What are the effects of a shallow and steep crown angle?

A

The have a negative effect on brightness and fire.

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15
Q

What techniques can you use to estimate crown angles?

A
  • Profile Method
  • Faceup Method
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16
Q

How does the girdle thickness affect a diamond’s durability and appearance ?

A

A girdle should be thick enough to prevent chipping but not so thick that it detracts from the stone’s appearance.

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17
Q

How doe you estimate girdle thickness?

A

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

18
Q

A weight-retention method that adjust the angles of some facets to allow portions of a diamonds girdle to be thicker.

A

Painting

19
Q

A weight-retention method where the cutter leaves more girdle thickness to allow for later removal of clarity characteristics around the girdle

A

Digging out

20
Q

Where is the focus when describing girdle thickness?

A

Valleys

21
Q

What facets are used to estimate the crown angle in profile?

A

Table and bezel facets

22
Q

The quality of the polish and precision of the cut of fashioned gemstones

A

Finish

23
Q

How do variations in pavilion dimensions affect a diamond’s appearance?

A

The pavilion makes a significant contribution to brightness and fire.

24
Q

How are pavilion depth percentage and pavilion angle related?

A

The greater the pavilion depth percentage, the steeper (greater) the pavilion angle.

25
Q

The distance from the girdle plane to the culet as a percentage of average girdle diameter.

A

Pavilion depth percentage

26
Q

The angle formed by the pavilion mains and the girdle plane.

A

Pavilion angle

27
Q

How does the girdle thickness percentage affect a diamond’s potential cut grade?

A

Girdle thickness can limit the cut grade.
>10% –> Lower cut grade
>= 11.5% –> Poor

28
Q

How do you estimate the length of a diamond’s lower half facets?

A

Estimate face up under 10x magnification and calculcate the average.

29
Q

What is the purpose of a diamond’s culet

A

The purpose of the culet is to prevent abrasion and chipping on loose diamonds.

30
Q

The length of the lower half facets express ad the percentage of the total distance between the girdle and the culet.

A

Lower half length percentage

31
Q

The size of the facet at the bottom of the diamond where the pavilion mains meet

A

Culet size

32
Q

How do design and craftsmanship influence a diamond’s cut grade?

A

Both are included in the cut grade.

33
Q

What effect does excess weight have on a diamond’s cut grade?

A

A diamond is overweight if it is heavier than expected by his (diameter) size.

34
Q

A diamond’s physical shape, including its proportions and durability, determined by the decisions made during the fashioning process.

A

Design

35
Q

The care that goes into the fashioning of a polished diamond, as confirmed by its finish

A

Craftsmanship

36
Q

What are the causes of some common polish characteristics?

A

They can be the result of the finishing process or by blemishes created after cutting.

37
Q

What are the quality ratings for polish and symmetry?

A

Both are rated seperately:
- Excellent (Ex)
- Very good (VG)
- Good (G)
- Fair (F)
- Poor (P)

38
Q

What factors determine a diamond’s overall cut grade?

A

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.

39
Q

How do parameters at the extremes in their range impact cut grade?

A

The limit the maximum grade

40
Q

How many crown angles on a round brilliant diamond should be estimated before averaging?

A

8

41
Q

What may result from a thick, unpolished girdle?

A

Grey reflections

42
Q

Why do diamonds with steep crowns offer less value?

A

They appear smaller than their weight suggests.

GIA Graduate Gemologist - GEM 230 (20 decks)

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