"Japanese buyers demand diamonds showing Hearts and Arrows" is commonly heard in the famous "Diamond Square Mile" of Antwerp, Belgium. Diamantaires have come to realise that not only the colour and clarity are important but also a perfect cut that displays hearts and arrows. This boils down to the question of what exactly is a perfect cut? The perfect cut is the one that brings out the maximum beauty in a diamond by way of life and fire. This has started into a massive controversy between various schools of thought each using a different computer model to analyse what proportions of a diamond cut would give the maximum life or brilliance in a diamond. This article is aimed at reviewing the various analyses of cut grades.

Diamond cuts have been developed through the ages starting from the 1st century BC and it has reached its zenith in the form of a round brilliant cut only in the 20th Century.

Optical properties is the fundamental aspect for bringing out the beauty of a diamond, in addition to its marvelous and most attractive body colour which human eye can perceive. The beauty of a cut diamond lies in its most superlative optical properties such as high refractive index, high degree of clarity, colour dispersion, reflectivity and lustre.

The quantity and quality of light that is reflected from the surface of a diamond is called lustre. Diamond has the highest quality of lustre viz. Adamantine. Other gems display lower qualities like vitreous, resinous, waxy, pearly, silky, dull etc. The quantity of light reflected from the surface of a diamond is only 17%. The remaining 83% light is refracted into the stone.

Fig.2
100% Reflection of light from the table ( 17% from lustre and 83% from reflection ) to total internal

When a diamond is cut and faceted into a Round Brilliant Cut (RBC) then it has crown facets and pavilion facets that are cut with certain precise proportion 83% of the light that falls on the front of a diamond is passed into it. The main objective and purpose behind the design of the modern brilliant cut is to make sure that the light entering into the diamond from the front side is not lost through the back and sides. The viewer therefore gets to observe 100% light from a RBC diamond (17% from lustre and 83% from total internal reflection). This quality of returning the maximum amount of light from the stone to the eye, from the surface lustre and from internal reflection, is known as "Life". In a cut diamond total internal reflections play a very important role. This total internal reflection is only possible when the light strikes the pavilion facets at an angle greater than the critical angle of the stone. In the case of diamond the critical angle is very low 24° (24° 26' ) and since the pavilion facets are cut at an angle of 40° , the light rays can strike at an angle

greater than the critical angle to cause return of light.

In the following diagrams a ray of light enters the crown of a brilliant cut diamond and that of a quartz of similar proportions. The light in both the stones strikes the pavilion facets at A in diamond and A, in Quartz, which is outside the critical angle. The ray is then totally reflected to B in diamond and B' in Quartz. In quartz the critical angle is greater than diamond i.e. 41 degree. The light falls within the critical angle cone in quartz and therefore does not undergo total reflection, but the ray is straight away refracted by the pavilion facets of the quartz into the air. On the other hand, in diamond that has a smaller critical angle (24° 26') the ray of light meets the point B outside the cone of the critical angle and undergoes total internal reflection. The ray of light reflected upwards meets the crown at point C which emerges as brilliance. All gems would display this phenomenon if they were to be cut with their pavilion facets at angles greater than their respective critical angles. Therefore the pavilion facets and the pavilion angle are responsible for the brilliance of the diamond.

The fire of a gem is the display of spectral colours caused by the refracting white light before returning it to the eye. The fire therefore depends on the dispersive power of the material.

When light is transmitted through a transparent prism shape it splits into seven colours, this is called dispersion. The spread of these spectral colours is dependent on the refractive index of the prism material; higher the index wider is the spread. The amount of spread of white light is called the dispersive power of the gem. In the case of an RBC diamond the crown facets act like small prisms that split the white light. Besides the refractive index of diamond being 2.42 and the dispersive power 0.044, rainbow colours observed from the crown facets of an RBC diamond are very prominent. It is the numerical difference between refractive index values of violet and red (2.451? 2.407 = 0.044). The dispersive power of cubic zirconia or the American diamond is 0.060 and therefore a cubic zirconia displays more flashes of colour as compared to a diamond. Hence crown facets and crown angle are responsible for the fire in a diamond.

The scintillating effect observed when a diamond is moved is the sparkle in a diamond. It is the number, intensity and frequency of flashes of white and coloured light. The quantity of light reflection generated depends on the number, size and symmetrical arrangement of the facets, as well as on the quality of the polish. The RBC diamond generally has 57 facets (58 including the culet) and every facet reflects light. The cut analyzers have pointed out that the number of facets should be increased with the size of the stone. If the diamond size is big the 58 facets are insufficient to give many scintillations. According to Rosch, (BRUTON, 1970) no facets should be longer than 3 mm or shorter than 0.5 mm to give good sparkle. Therefore stones over 10 carats should have more than the usual 58 facets and those that are smaller than 0.12 carats should have fewer facets. Therefore small stones often have the eight cut. This explains why diamonds like the Jubilee (245.33 carats) has 88 facets, the King 86 facets and the Cairo Star has 74 facets (BRUTON, 1970).

The life, fire and sparkle are best brought out by the round brilliant cut. The superior optical properties of a diamond especially, its high refractive index, dispersive power, its lustre are fascinatingly displayed by the brilliant cut. PAGEL THEISEN, 1970 reports that in a cut diamond all the factors are in operation which in optical harmony with one another create the highest degree of brilliancy. Maximum brilliancy is the interwoven concord of lustre, light refraction, total reflection, colour dispersion and scintillations. The knowledge and application of these physico?optical laws have gone into the development of the brilliant cut. Only when precisely calculated planes and angles are used in the brilliant cut does the diamond attain its greatest possible beauty. These precisely calculated planes and angles are called the cut proportions of diamond.

In the brilliant cut the proportions are of prime importance. The greatest dimension is the girdle and is considered as the 100%. All other diamensions are considered as the percentage of the girdle viz. Table percentage, Crown height, Pavilion depth, Girdle thickness, Total depth. The angle made by the crown facets with the girdle is called the crown angle and the angle made by the main pavilion facet with the girdle is called the pavilion angle. Since 1910 theoretical calculations have been employed to achieve maximum brilliance through suitable proportion and symmetry ratios (PAGEL THEISEN, 1970).

In the year 1919, Marcel Tolkowsky published his book Diamond Design and stated that, "the most vivid fire and the greatest brilliancy" is obtained with pavilion main angles 40.75° crown main angles 34.5° with a table size of 53%, crown height 16.2% and pavilion depth of 43.1%. These proportions have been considered by the GIA as the American "Ideal" cut.

The AGJ Japan considers "Excellent" cut to have pavilion main angles between 40.0° and 41.3° crown main angles between 33° and 35° with a table size between 53% and 58%, total depth of between 59.2% and 62.4% and having pavilion depth of 42% and 44%.

In GIA, Tolkowsky's cut is considered as the best and therefore the students have been taught that "even a two degree deviation from Tolkowsky's theoretical pavilion angle will result is a less attractive diamond". An increase of two degrees in the pavilion angle, i.e. instead of 40.75° if the angles are between 43° and 45° it will result in a noticeable darkening of the stone and an obvious loss of brilliance that gives rise to "nail head" effect. Since this gives rise to a deep pavilion, much of the light is leaked out, then the table reflection and the star facets look almost black.

Depth of Pavilion is decisive for the reflection of light Long pavilion facet gives a noticeable darkening of stone and give rise to nail head effect.

Fig.9

Increase in crown and angle and decrease in table size give rise to more fire & less life.

The Cut grade plays a very important role in the evaluation of diamonds. Maximum Life, Fire and Sparkle are observed in diamonds that are cut and faceted to certain specific proportions. The ideal proportions for the cut grade have been reviewed from different grading standards. A REFERENCES new concept developed called Weighted Light Return (WLR) shows which type of cut gives the maximum brilliance. The famous Tolkowsky's cut although very good for displaying optimal dispersion, brilliance and scintil lations gives a much lower value of WLR as compared to others. A perfect symmetrical cut gives the eight Hearts and Arrows when observed through special devices ? it's quick method to check whether the diamond is synimetncally cut.

It is a known fact that every eight out of ten diamonds in the world are faceted in India. Therefore it is the need of the hour that diamond cutters take extra care to give a perfect cut so that not only quantity but also quality of the cut is maintained to keep India as numero uno in diamond cutting.