The Diamond, not only brilliant cut

The best known shape for diamonds is round, the so-called “brilliant”. The common association of the term “brilliant” to the diamond stone comes from its typical cut style with many facets (57 in the round cut) that ensures maximum brilliance.
Round brilliant cut:

Other types of cut:

The cut of a diamond is defined by the following criteria:
SHAPE: round, oval, drop, square etc.
STYLE: brilliant or stepped, rose, mixed etc
CUT GRADE: the evaluation dipends of the proportions, symmetry and the so-called polishing.
A diamond’s brilliance depends mainly on its cut: the greater the accuracy of the cut according to ideal proportions, the greater the lustre of the stone.

An ideal cut allows total internal reflection and therefore the maximum emission of light through the crown.
Brilliance is defined as the amount of light that the stone is capable of transmitting to the observer’s eye. The light transmitted from the gem is the result of refraction and internal reflection of the penetrating rays that, guided in their optical path by the angle of the facets, bounce back through the crown.

Proportions
With the aim of obtaining the maximum brilliance, brilliant cuts with different proportions have been studied, among which the Tolkowsky’s cut that is closest to the ideal proportions.
Cut classification is based on deviation from these proportions.
The basic data for judging the value of a cut is obtained by measuring the height of the crown and the pavilion, the thickness of the girdle and the extension of the table. These measurements give a mathematic representation that allows easy evaluation, if reported in percentage, to the average diameter of the girdle.
The proportions of the round cut brilliant are carried out using an optical instrument designed for that purpose, called a diamond proportion analyzer.

Symmetry
The symmetry of a gemstone evaluates the alignment of the cut elements such as facets, the culet and the mirroring from one side of the gemstone to the other.
A visual analysis of the gemstone can highlight asymmetries like the presence of off-centred tops compared to the centre of the table or a different dimensioning of the facets.
The analysis of symmetry, such as the proportions, affects the evaluation of the cut.

Polishing
The term polishing refers to the polishing of the facets of the gemstone. The presence of scratches and grooves are a proof of a bad finish. The analysis of polishing is therefore a factor which affects the value of the cut of the gemstone.