GRADING OF ROUND DIAMONDS ACCORDING TO TU N 25-07.1319-77
Classification of diamonds by color

Size

Color

Grade

Kr-17

Colorless

1

Slight yellowish, yellow, green, aquamarine, or gray tint or with slight brownish shade

2

Clear yellow tint and yellow stones with slight brown tint

3

Brown

4

Êð-57

Êr-57 Small stones (no more than 0.29 ct)

Colorless

1

Slight tint

2

Slight yellowish, aquamarine, green, violet, gray, or slight brown tint

3

Clear yellow, lemon, green, aquamarine, or gray tint

4

Yellow, green, or lemon color of the whole diamond

5

Slight brown color

6

Brown or yellow-brown color

7

Median and large (more than 0.30 ct)

Colorless white and with bluish tint

1

Colorless

2

Slight trace of color

3

Slight yellow tint

4

Slight yellowish, greenish, aquamarine, violet, or gray tint or slight brown shade

5

Clear yellow, green, aquamarine, gray, or slight brown tint

6

Clear yellow, green, aquamarine, gray, or lemon tint

7

Yellow, green, ore lemon color of the whole diamond

8

Brown or yellow-brown color

9

Notes:


Diamonds with unique colors (blue, pink, emerald-green, and other rare colors) should be assigned to color grade 1.

Diamonds having black color when observed face-up (because of "graphite" inclusions) should be assigned to the last color grade.

Correspondence between diamond weight and cutting style.

Kr-17

up to

0.040 ct

Kr-57

more than

0.005 ct


Clarity grading

 

Characteristics of clarity grades

Clarity grade

Kr-17 Kr-33

Kr-57

Up to 0.30 ñt

More than 0.30 ct

Flawless

1

1

1

Defects: one light point, in the central zone only visible from the pavilion side or no more than two slightly visible light points or no more than one slightly visible light line in the middle or girdle zones.

2

2

2

Defects: no more than three minor light points in the central zone or no more than two minor dark points or lines in the middle or girdle zones

3

3

Defects: no more than two minor dark points in the central zone or no more than four small light points in any zone or no more than two lines in any zone or no more than one line and three small light points in any zone or one minor crackin the girdle zone

4

Defects: central zone: one small light cloud or one minor crack or no more than six small light lines; middle or girdle zone: no more than three minor cracks

3

4

5

Defects in any zone: no more than eight small scattered light points, lines, minor cracks, bubbles, twinning lines, or growth lines or no more than five small dark points or no more than one minor "graphite" inclusion

5

6

Defects in any zones: no more than eight small scattered defects (including those slightly visible to the naked eye): points, lines, minor cracks, dust clouds, or one "graphite" inclusion.

7

Many defects in any zone: no more than two small "graphite" inclusions or no more than two minor cracks or one small cloud and one "graphite" inclusion or one small crack and "graphite" inclusion or several minor cracks with one "graphite" inclusion.

7a

Many defects in any zones: any features except for "graphite" inclusions, including cracks visible to the naked eye.

4

6

8

Many defects in any zones: "graphite" inclusions or "graphite" inclusions combined with cracks visible to the naked eye.

9

In any zones: various defects visible to the naked eye; at least 60% of pavilion facets are transparent to observer's eye

7

10

In any zones: various defects visible to the naked eye; less than 60% of pavilion facets are transparent to observer's eye

5

8

11

Imperfect

6

9

12

Notes:


Diamonds more than 0.30 ct in weight with an extra facet or culet cannot be assigned to the first clarity grade.

Defects in diamond include morphological features (cracks, points, lines, twinning lines, bubbles, clouds, growth lines, inclusions) inherited from the original diamond, mechanic defects produced in the course of cutting, and reflections of defects visible through the table.
Upper and lower girdle facets
The height of the upper girdle facets when measured in plan, should account for 40-50% of the crown height. The height of the lower girdle facets should account for 75-90% of the pavilion depth, and it should be the same for a particular diamond. The junctions between the main and girdle facets of a diamond should be sharp.
Natural
The pavilion facets of diamonds of the Kr-17 and Kr-57 cutting styles no more than 0.29 ct in weight must have no more than four naturals and extra facets, and those of diamonds 0.30 ct and more in weight - no more than two naturals or one natural and one extra facet. The natural surfaces may have morphological features inherited from the faces of the original diamond crystal without significant steps and etch figures. Naturals and extra facets should not affect the girdle shape in plan and should be observable through the table in the direction parallel to the symmetry axis of a diamond.
Girdle
The girdle of a diamond should be regular in shape and even in thickness within accepted tolerances. The girdle surface should be clean, smooth, and mat-gray and should correspond to the girdle surface of a masterstone. The girdle thickness may vary within 25% between its minimum and maximum values. For diamonds more than 1.00 ct in weight, girdle should visually have the same thickness over its perimeter. The girdle thickness in places where naturals were left should be less than 50% of the girdle thickness above the adjacent upper or lower girdle facets.
Girdle irregularity

Cutting style

Weight (ct)

Microcracks are not tolerated

Clarity grade

Color grade

Kr-17

Up to 0.04

1 - 2

1 - 2

Kr-57

up to 0.29

1 - 3

1 - 3

0.30-0.49

1 - 4

1 - 6

0.50 and more

1 - 7

1 - 6


Diamonds with other quality parameters may contain microcracks extending into the stone for no more than 0.05 mm, if the cracked zones (in total or separately) do not exceed 1/3 of the girdle perimeter.

Diamonds of Group "B" more than 0.03 ct in weight may contain up to ten loupe-visible (10x) microcracks per carat over the girdle perimeter, with the cracks extending for no more than 0.07 mm into the stone.

Culet


The culet size should not exceed 2% of the diameter or width of a diamond less than 0.49 ct. in weight and 1% of the diameter or width of a diamond more than 0.50 ct. Diamonds more than 1.00 ct. in weight may have a pointed culet slightly visible at tenfold magnification.

The shift of the culet and the table center from the symmetry axis of a diamond up to 0.29 ct. in weight is tolerated within 2% of the diamond diameter or width. For diamonds ranging from 0.30 to 0.99 ct. in weight, the tolerance value is established at 1.5% of the diamond diameter of width.

Parameters of polished diamonds


Diamonds with any quality characteristics are fashioned according to A-group parameters.

B-Group parameters are applied to:

Diamond size

Clarity grade

Color grade

Diamonds Êr-17

3 - 5

1 - 4

Diamonds Êr-17;

small, up to 0.29 ct. in weight

4 - 8

1 - 7

1 - 3

4 - 7

Diamonds Êr-57;

medium, from 0.30 to 0.49 ct in weight

5 - 10

1 - 9

1 - 4

6 - 9

Diamonds Êr-57;

; medium and large; more than 0.50 ct in weight

1 - 10

8 - 9

11

 
Correlation between table size, pavilion height and pavilion angle
Parameters of Kr-17 and Kr-57 diamonds

 

Group

D,ìì

d,%

optimal (tolerance limits)

h,%

optimal (tolerance limits)

a ,ãðàä.

optimal (tolerance limits)

b ,ãðàä.

optimal

Kr-17

À

up tî 2.2

60 (55-65)

2.0 (1.5-3.0)

33 (30-35)

41

B

up tî 2.2

60 (60-65)

2.0 (1.5-5.0)

33 (30-40)

41

Kr-57

À

to 5.2

60 (55-65)

2.0 (1.5-3.0)

33 (30-35)

41

from 5.2

60 (55-65)

2.0 (0.7-2.0)

33 (30-35)

41

up to 6.5

60 (55-63)

2.0 (0.7-2.0)

33 (32-36)

41

Á

-

60 (50-65)

2.0 (0.7-5.0)

33 (30-37)

41

Correlation between table size, crown height and crown angle
Facets


The crown facets should be exactly above the pavilion facets. For diamonds no more than 0.99 ct. in weight, the shift of the crown edges relative pavilion edges is tolerated within:

10% of the base of the upper girdle facet or base of the bezel facet for A-group diamonds;

20% for B-group diamonds

All the cutting elements of the crown of a diamond more than 1.00 ct. in weight must be visually symmetrical to those of the pavilion.

Curved facets and edges and chips visible at tenfold magnification are not tolerable.

The facet surfaces should be carefully polished. The surfaces must not show any traces of graphitization, , lines, scratches, and other fashioning defects visible at tenfold magnification, except for diamonds weighing no more than 0.29 ct. (clarity grade 8) and diamonds weighing more than 0.30 ct. (clarity grade 10) for which removing the features cannot improve the clarity grade.

If a diamond contains internal natural defects (twinning lines, inclusions, cracks), their traces may be present in any part of the surface as points or lines, provided the boundaries of the defects are not chipped off.

The variation of the crown or pavilion angles of a given diamond is tolerated within

- 1° for Group "A",

- 2° for Group "B".

For diamonds more than 1.00 ct. in weight, the variation of the angles between the upper girdle facets and the girdle plane is tolerated within 1°.

Tolerances for the oval girdle in dependence on diamond diameter

Round diamonds should have a circular shape over the girdle perimeter when observed face up at the right angle to the table.

Tolerable deviations of the oval girdle from the round shape:

Group "A"

Group "B"

Up to 3.0 mm

- 0.06 mm

Up to 3.0 mm

- 0.08 mm

from 3.0 to 5.0 mm

- 2% of D

from 3.0 to 5.0 mm

- 2.5% of D

more than 5.0 mm

- 0.10 mm

more than 5.0 mm

- 0.13 mm