Diamonds - Ideal Cut Diamonds

Australian International Diamond Exchange

Melbourne Diamond Exchange ltd

Icestore (Australia)

Diamonds - Diamond Information

Proportions

The brilliancy and fire of a diamond are largely determined by the angles of inclination and proportional relations between the various parts of the stone. If these are not optimal, specific undesirable effects can occur. When grading the proportions of a polished diamond, the main issue is therefore to evaluate if, and if so to what extent, these effects occur. The most important effects that can be perceived when observing the stone perpendicular to the table are:

fish eye :	the reflection of the girdle is visible through the table
black table reflection :	the reflection of the table is so large that the table appears to be almost completely black
culet visible in bezels :	the stone shows an abnormal amount of scintillation, due to the culet and the surrounding facets being visible through the bezels
window :	the light falls through the stone and leaves it at the bottom without any reflection
single cut :	the diamond looks as if it has less facets than it actually has because the difference in the angle of inclination between the facets is too small (no sharp edges)

Diamonds - Proportion of Diamond - Ideal cut

Diamonds - Proportion of Diamond 2 - Ideal Cut

The list of effects above is not exhaustive; there are other proportions -related phenomena which are considered to be undesirable, for instance the diameter being too small in proportion to the total depth, making the stone appear smaller than its actual weight.

For the brilliant- shapes, next to the actual values an appreciation can be given :very good / good / unusual The measurements of the different parts of the stone can be a useful aid in determining the proportions grade, since there exists an obvious relation between these data and the appearance of undesirable visual effects.

Criteria	Unusual	good	very good	Good	unusual
crown angle (ß) pavilion angle (a)	up to 26.9° up tp 38.4°	27.0° to 30.6° 38.5° to 39.5°	30.7° to 37.7° 39.6° to 42.2°	37.8° to 40.6° 42.3° to 43.1°	40.7° and up 40.7° and up
table width (% Ø_t)	71 and up	70 to 67	66 to 53	52 to 51	50 and down
crown height ( % h_c)	up to 8.5	9 to 10.5	11 to 16	16.5 to 18	18.5 and up
girdle thickness (% a)	extremely thin 0 to 0.5	very thin 1 to 1.5	thin & medium 2 to 4.5	thick & very thick 5 to 7.5	extremely thick 8 and up
pavilion depth ( % h_p ) (for pointed culet)	up to 39.5	40 to 41	41.5 to 45	45.5 to 46.5	47 and up
culet size (%)	---	---	pointed to 1.9 %	2% to 3.9%	4% and up
total depth (% t_d )	up to 52.9	53.0 to 55.4	55.5 to 63.9	64.0 to 66.9	67.0 and up

If the readings for a stone are situated in different categories, the lowest grade is considered to be the overall reading. In addition to the factors mentioned above, there are other proportions which can have a negative influence on the final grade if they are not within certain limit, such as for example the height of the crown and on the pavilion.

The weight of a diamond is expressed in carats. 1 carat equals 0.2 grams. A carat is further subdivided in 100 points. The diamond -is weighed immediately upon receipt at the reception desk. The HRD has extremely accurate scales, up to 1/100,000 of a gram. This weight is converted into carats and rounded off. On the receipt and return notes, the weight is mentioned up to five decimals. This accurate weight can be very useful in identifying the stone. On the certificate, the weight is mentioned up to two decimals, e.g. 1.50 ct=1 carat and 50 points.

expressed in carats : 1 ct=0.2 gram
- two decimals are mentioned
- the third decimal is rounded as long as long as it is smaller than 9

e.g.	measured		mentioned
	1.234 ct	>	1.23 ct
	1.239 ct	>	1.24 ct

Fluorescence is the effect whereby invisible ultraviolet light is tranformed into visible light. Some stones light up spectacularly in the dark when irradiated by long-wave ultraviolet light. The colour of the fluorescence is usually blue, but other colours are possible too.

Normal daylight contains a certain amount of UV-light, and therefore the fluorescence can influence the colour of the stone. The degree of fluorescence is determined by comparing the diamond to a series of masterstones under long-wave UV-light ( wavelength 366 nm). The fluorescence grades are: nil, slight, medium and strong.
The colour of the fluorescence is not mentioned on a certificate.

The shape is the form in which the stone is polished. The round brilliant is the most popular and well-known diamond shape, but also the pear, the oval, the marquise, the heart and the emerald cut are much in demand. Numerous other fancy shapes exist. On the certificate, the name of the shape is mentioned next to the heading in the middle part. On the left side of the certificate a drawing of the basic shape is included

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