COLOR GRADING

Diamonds which have no color (absolutely colorless) are especially valuable. However, most of gem-quality diamonds have tints of various intensity. All diamonds can be conditionally divided into two groups: more abundant yellow and yellow-tinted diamonds (cape series) and diamonds with fancy colors. Precise color grading of cape diamonds is carried out with a set of color masterstones, for instance, the well-known GIA set (from D to Z). The color scale is subdivided into ranges, beginning with absolute colorlessness, which gives way to zones with a gradually increasing intensity of yellow tint and, at last, to the marginal group with a clear yellow color. According to the GIA system, each grade of this color scale is denoted by a letter, from D (colorless) to Z (yellow). The same scale can be used for grading stones of brown series, with only intensity rather than tint of a stone evaluated. Yellow and brown stones having a color below Z are considered to have fancy colors. The table demonstrating the GIA system of color grading is given hereon the next page.


Requirements for masterstones
  • The weight of a masterstone should not be less than 0.25 ct. Masterstones in a set should have approximately the same weight.
  • The weight (size) of masterstones should be as close as possible to the weight (size) of an unknown diamond.
  • Diamond masterstones are preferable, although those made from phyanite (cubic zirconia) can also be used.
  • The masterstones should be pure yellow, without any other tints.
  • The masterstones should not contain any colored inclusions, inclusions visible by the unaided eye, or defects affecting color (at least SI1 quality).
  • Ultraviolet fluorescence should be very weak or absent.
  • Cut style: round brilliant.
  • Cut, proportions, symmetry, and polish of the masterstones should be of a high quality.

Any color represents a color range; hence, the masterstones are calibrated against either light or dark boundaries of the color ranges.


GIA system of color grading
GIA Color Grades Descriptive Terms More detailed description
D Finest White Colorless
E
F Fine White
G
H White
I Commercial white Stones in these grades will face up colorless( i.e. slight traces of color will not be apparent in mounted stones to other than the trained eye.
J Top Silver Cape
K Silver Cape
L
M Light Cape Small stones in this range will face -up colorless when mounted but large ones will be tinted
N
O Cape
P
Q
R
S Ц Z Dark Cape

Other adopted international grading systems use characteristic words instead of letters for description and evaluation of colors. Certificates prepared in accordance with one grading system often contain tables enabling conversion of all the color grading terms into other systems.


Comparison table of different colour grading systems
    (From: Verena Pagel-Theisen. "Diamond Grading ABC").
GIA CIBJO 1974 RAL 560 A 5 "Old Terms" Scan D.N.
German Name International Name
D Hochfeines Weiss

(Finest White)

Exceptional white; blanc exeptionnel River (R) River
E
F Feines Weiss

(Fine White)

Rare white

Extra blanc

Top Wesselton (TW) Top Wesselton
G
Weiss

(White)

White

Blanc

Wesselton

(W)

Wesselton
H
I Leicht getontes Weiss

(Slightly Tinted White)

Slightly tinted white

Blanc nuance

Top Crystal

(TCA)

Top Crystal

J
K Getontes Weiss

(Tinted White)

Tinted white

Legerement teinte

Crystal (CR) Crystal

L Top Cape(TCA) Top Cape

M  

Gelblish

(Yellowish)

 

Yellowish

Teinte

Cape

(CA)

Cape
N
Light Yellow (LI) Light Yellow
O...Y
Z Farbige Diamanten

(Coloured Diamonds)

Fancy diamonds

Diamants de couleur speciale

  ‘антазийные цвета

If a stone has any tint other than yellow, the color grade is determined within the same scale with indicating the relevant color. As a rule, if a diamond is near colorless (better than H), its tint is hardly definable and is usually not indicated in the worksheet. Stones with a deeper color than H-grade are visually tinted, and the color grade can be indicated in the worksheet or certificate, especially if this tint is not yellow.

Special masterstones are used for fancy colors.


Description of the color grading procedure
  • The surfaces of the determined diamond and masterstones should be clean.
  • It is recommended to note the identification features of a diamond under study, such as distribution of characteristics and luminescence.
  • The diamond color should be graded against the background of the standard white paper sheet, under standard day-light illumination free from ultraviolet rays, and in a darkened room.
  • The masterstones should be set from the left to the right (from the lightest to the darkest), at a small distance from one another, table down.
  • The masterstones should be set so that expert's eye is directed either at the right angle on the pavilion facets or in parallel to the girdle plane.
  • The unknown diamond should be moved along the series of the masterstones, comparing the color of its pavilion or other zones of a similar size if the evaluated stone and masterstones differ in weight.
  • The evaluated stone should be held as close as possible to a masterstone, but without touching it.
  • The evaluated stone should be compared to a masterstone from its both sides.
  • As the color masterstone is found that best fits the color of the unknown diamond, the color grade is established for the specimen.
  • If the masterstones correspond to the light boundaries of color ranges, the evaluated stone will have the color of a lighter masterstone.
  • If a diamond contains large or/and colored inclusions (structural features), the possible effect of these defects on the stone color should be estimated. The ultimate decision regarding the color grade should be referred to the zones of the stone free from any inclusions (structural features).
  • In the end of the grading procedure, one must be sure that the graded diamond is not confused with a masterstone.

In addition. Because of some peculiarities of human's eye, even two stones of the same color can be perceived in a different way. Hence, to ensure the correctness of the color grading, a diamond should be examined on both sides of a masterstone. The diamond can be inferred to be identical in color to the masterstone if the former and the latter alternately show a deeper color on interchanging their positions.