INSTITUTO GEMOLOGICO ESPAÑOL

XXVIII INTERNATIONAL

GEMMOLOGICAL CONFERENCE

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G E M M O L O G I C A L C O N F E R E N C E

X X V I I I I N T E R N A T I O N A L

G E M M O L O G I C A L C O N F E R E N C E

X X V I I I I N T E R N A T I O N A L

G E M M O L O G I C A L C O N F E R E N C E

X X V I I I I N T E R N A T I O N A L

G E M M O L O G I C A L C O N F E R E N C E

X X V I I I I N T E R N A T I O N A L

G E M M O L O G I C A L C O N F E R E N C E

Madrid - 2001Madrid - 2001Madrid - 2001Madrid - 2001

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D E F G H I J K L M N O-R S-Z

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FL IF

VV

S1

VV

S2

VS

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VS

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SI1

SI2 P1

P2

P3

Carat weight Colour

Cut

Other characteristics

ConclusionsStudy of the properties of colour D diamonds

Clarity

The distribution of analysed diamonds by their weights is very irregular and shows well defined maximums at nearly round weights (for example, 1.0-1.1, 0.5-0.6 weight groups), related with the diamond price changing at this points.

The majority (90,26%) of all diamonds have round brilliant cut. Other cut types and shapes as well as basic round br i l l iant cut proportions and other cut parameters were studied.

The ideal values of two different round brilliant cut models (Tolkowsky and Tillander) are also plotted for comparison with the data given by real diamonds.

Comments:1 - The comparison of the obtained data on the types and shapes of cut with the selection of 500 diamonds analysed by IGE lab at 1970 shows the increasing of “non traditional” (not round brilliant) cuts percentage.

2 - Overage girdle width distribution for all diamonds sizes is represented in the figure. We also have studied the overage values of girdle for different weight groups, both in diameter percent and in mm absolute values:

Other characteristics which are systematically archived for analysed diamonds are UVL (365 nm) fluorescence (colour and intensity) and the intensity of 415.5 nm line in optic absorption spectrum.

Visual estimation is used for both characteristics, defining 5 fluorescence intensity and 7 optic absorption grades.

The study of 85 colour D diamonds (1.63 % of all analysed stones) were carried out to determine the correlation between their fluorescence and optic absorption spectra. General distribution of values fluorescence intensity and 415 nm absorption line are very similar with those represented above for all colours diamonds. But, in contrast for colour D diamonds some correlation of this values was observed (see 3D figure below). One can see that colour D diamonds tend to plot 2 families of values on this diagram. First one characterises by the absense of absorption at 415 nm and very weak to medium fluorescence (blue columns, 42 stones, 53 % of all colour D diamonds). These are the diamonds which do not below to the Cape type. Other group of colour D stones (47 %) is formed by Cape type diamonds generally with well pronounced absorption at 415,5 nm and strong ore medium fluorescence.

Distribution of analysed diamonds by their colour grade, obtained as a result of comparation with diamond colour masterstones. In 101 diamond (1,9 % of all diamonds) rare non-compartible or fancy colours were observed.

The majority of analysed diamonds has the VS2 clarity grade. No P3 diamonds were observed in the last 5200 certified stones.

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0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1 1,1 1,2 1,3 1,4 1,5 1,6 1,7 1,8 1,9 2 2,1 2,2 2,3 2,4 2,5 2,6 2,7 2,8 2,9 3 3,1 3,2 3,3 3,4 3,5

carat weight

diamonds analysed diamonds analysed

colour grade clarity grade

diamonds analysed

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very weak weak medium strong very

strong

blue, 51.81 %

bluish, 46.99 %

greenish blue, 1.2 %

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Statistic study of characteristics of diamonds certified by IGE gem testing laboratory

brilliant, 93.04 %

emerald, 3.91 %

princess, 1.57 %

old cut, 1.08 %

others, 0.41 %

(see comment 1)

round, 91.76 %

rectangular, 4.46 %

square, 1.1 %

oval, 0.58 %

marquise, 0.58 %

hart, 0.39 %

pear, 0.85 %

triangle, 0.14 %

others, 0.14 %

rough, 57.93 %

faceted, 37.81 %

polished, 3.68 %

semipolished, 0.41 %

partly faceted, 0.17 %

pointed, 82.65 %

polished, 9.47 %

slightly damaged, 7.79 %

natural, 0.09 %

very good, 10.85 %

good, 82.43 %

medium, 5.67 %

deficient, 1.04 %

very good, 27.23 %

good, 70.20 %

medium, 1.68 %

deficient, 0.89 %

Cut type

Girdle type Culet type

Symmetry Polish grade

Shape

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7,5 8 8,5 9 9,5 10 10,5 11 11,5 12 12,5 13 13,5 14 14,5 15 15,5 16 16,5 17 17,5 18 18,5

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1 1,5 2 2,5 3 3,5 4 4,5 5 5,5 6 6,5 7 7,5 8

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Crown height

Culetdeviation

Tabledeviation

Girdlewidth

(see comment 2)

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53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72

Tablewidth

blue, 22.34 %

bluish, 73.00 %

yellowish, 0.28 %

greenish blue, 1.91 %

yelloweesh blue 1.01 %

others 1.47 %

Fluorescence colour

UVL fluorescence intensity

UVL fluorescence colour

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very weak weak medium strong very

strong

UVL fluorescence intensity

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Optic absorption spectra(415,5 nm line intensity)

Optic absorption spectra(415,5 nm line intensity) 2

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8 5

29

5 5

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2

6very weak

weakmedium

strong

very strong

415,5 nm absorption line intensity

very strong UVL flu

orescence

strong

mediumweak

very weak

very very weak

not observed

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Destribution of colour gradesfor all diamonds with

very weak fluorescence and without absorption line 415,5 nm

Destribution of colour gradesfor all diamonds with

strong luminiscence and medium absorption line 415,5 nm

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38,5 39 39,5 40 40,5 41 41,5 42 42,5 43 43,5 44 44,5 45 45,5 46 46,5 47 47,5 48 48,5

Paviliondepth

57,5 % - idealof Tillander model

14,6 % - idealof Tillander model

43,1 % - ideal

and Tillander modelsof Tolkowsky

53 % - idealof Tolkowsky

model

16,2 % - idealof Tolkowsky

model

The results of the statistic study of the last 5200 diamonds analysed by IGE gem testing laboratory are presented. Such characteristics as carat weight, colour and clarity grades, basic round brilliant cut proportions, UVL (365 nm)

fluorescence and absorption spectra characteristics (the intensity of the 415,5 nm line) were taken into account.The relation between the UVL fluorescense and optic absorption was studied for colour D diamonds.

Weight: Girdle, % Girdle, mm

0.2 - 0.8 ct 3 - 3.5 0.22 - 0.24

0.81 - 1.45 ct 2.5 - 3 0.24 - 0.26

1.451 - 2.9 ct 2 - 3 0.26 - 0.28

2.91 - 10 ct 2 - 2.5 0.3 - 0.32

All weights 2.5 - 3 0.24 - 0.26

The distribution of weight of analysed diamonds is very heterogeneous. The majority of diamonds belongs to the 1-1,1 or 0,5-0,6 ct weight groups.

The most frequent grades of colour and purity of certified diamonds are H and VS2.

90 % of all diamonds have round brilliant cut. Other most used cuts are: emerald (3.91 %) and princess (1.57 %).

The real values of pavilion depth and table width of round brilliant cut are usually bigger than ideal ones, while the crown height is usually smaller.

Colour D diamonds represent 1,63 % of all diamonds analysed. This colour grade can have both Cape type diamonds and diamonds which do not below to Cape type.

Two groups of diamonds were find to have a great probability for high colour grades:- diamonds which have no absorption line at 415,5 nm - those which have quite strong absorption at 415,5 nm but also medium UVL fluorescence

Egor V. GavrilenkoJuan S. Cózar