Image showing the variety of colour and cut in diamonds.

Colour, Carat, Clarity and Cut... these are the four important characteristics by which gemstones (especially diamonds) are judged.  Of the four, cut is the most important one, the one that brings the other three qualities together.  Sacrifice cut for carat weight and you have a stone with poor colour diffusion and bad light reflecting properties.  If the stone is not studied properly and the inclusions are not well disguised within a stone, the value plummets.

 A cutter marks a stone with Indian ink before starting work on a stone.

The cut of a stone is only really an exact science where diamonds are involved.  Diamonds have a high refractive index.  The modern, 58 facet brilliant cut was perfected in 1919 by Marcel Tolkowsky and it was developed to reflect the maximum possible amount of light entering a diamond.  The number of facets, what angle they should be at and the proportions of the stone were meticulously, mathematically worked out to achieve this.  The modern brilliant cut, bar some minor refinements, remains intact today as no-one else has been able to improve on it.  If the proportions are altered, the light enters and leaves the stone in the wrong way and it will appear dull and lifeless. That is not to say that brilliant is best:  different cuts produce different effects and buyers always should buy what they like most.  Some years ago, there was a trend amongst dealers selling diamonds that had a greater ‘spread’: they were cut shallower to make them appear larger, but in doing so were sacrificing the brilliance of the stone. 

The most commonly used cuts in diamonds (back to front, left to right): Marquise, heart, princess, emerald, marquise, asscher, brilliant, pear.

Diamonds, as we all know, more often than not contain some trace of impurity that will interfere with clarity and the lower the clarity, the lower the value.  If the cutter cannot remove the stone’s flaws, he must work with it to leave it where it will interfere least with the play of light.  Inclusions in diamonds should never really be able to be seen through the table (the largest facet on top of the stone).   In a brilliant cut, they should be nearer the edge of the stone, where it will be less visible under the criss crossing of the facets.  This is far harder to achieve with emerald cut stones, as the parallel nature of the cuts is unforgiving and flaws are far more readily visible, so these stones tend to be of a higher clarity.  However, there should be no inclusions visible on a girdle (the thin edge round the middle of the stone) as these are a structural weakness in the gem and where the stone is more likely to crack if handled wrong by the setter.  It is always better to sacrifice weight and remove inclusions as a much higher price per carat will be achieved for the stone.

Exceptional quality rubies.

Mathematical formulae are not so important with coloured diamonds and gemstones.  In comparison to diamonds, most coloured gems are heavily included and the cutter must cut the stone for it to appear as saturated in colour as possible.  It is rare for a coloured gem to be consistently coloured- it tends to concentrate in a part of the stone in an effect known as zoning.  It will be the cutters’ job to study each gem individually and cut the stone in such a way that when viewed from above the colour appears even.  Colour also often has hues from other parts of the colour spectrum- emeralds can appear bluish green, rubies, purply red, etc.  A skilled cutter can eliminate these unwanted visual intrusions.  Again, the incidence of inclusions has to be taken into account and ensure that they don’t arise where they might structurally compromise the gem.

In my opinion, stone cutters are often the unsung heroes of the jewellery world.  They have to contend with all the above variables to achieve perfection and no amount of theory can make them good at what they do- it requires thousands of hours of practice.  A rough gem of importance will take many, many months of study before the first cleave is made and its full potential unveiled. 

Beautiful blue sapphires: the evenness of colour will have been achieved partly by the stone cutter.