The work of the lapidary (facet cutting)
Two interesting facet cutting articles in English from Hubert Heldner and International Gem Society:
What is the average gemstone faceting yield?
Perfect faceting angles versus excess weight
The purpose of faceting for coloured stones is to give the raw material an optimal performance in terms of brilliance.
It is therefore necessary to analyse and respect the characteristics of each stone, in particular the optical axis, the pleochroism and the possible colour zones.
The lapidary will always be confronted with the cornelian choice to maintain a higher carat weight, therefore more interesting from an economic point of view, even if it means letting more or less visible inclusions appear or reducing the stone as much as possible in order to opt for a high purity.
– First step:
Choose a rough stone for its colour, it must be neither too light nor too dark, its purity by scrupulously identifying the inclusions that are too disturbing as well as the cracks and also for its shape which must not be too flat.
– Second step:
Find a diagram of suitable size so as not to lose too much material. However, expect a loss of about 70% on average! Precision cutting techniques used today do not put weight first but optical performance. The sizing diagrams that have been used for a number of years have been developed with the help of software in order to obtain the best possible brilliance.
– Third step:
Begin to preform the stone to eliminate the areas that are too included, take into account the axis with the best colour and give it a shape close to the chosen size diagram. (pear, oval, rectangle, cushion,…).
– Fourth step:
Diagramme: source: H. Heldner
Start the actual facet cutting by adapting the diagram to the refractive index of the stone in order to optimize the brilliance. It is indeed the angle best adapted to the type of stone that will give a better reflection of the light and avoid taking the “window” in the centre of the stone synonymous with poor quality cutting.
Example of an Aquamarine, badly cut on the left with a window and correctly cut on the right.
(Photos: International Gem Society).
Do not confuse reflection and refraction. Refraction refers to the index of refraction (IR) of a stone, which is the ratio between the speed of light in air and the speed of light inside the stone. The IR is calculated using a refractometer. But it is by respecting this index that the reflection of light will be the best.
The perfect respect of the junction of the facets will also contribute to the quality of the cut.
– Fith step étape:
Polishing marks the end of the cutting process and must be carried out with precision in order to remove all micro scratches and make the stone as shiny as possible. Remember to clean the stone gently with a solvent to remove any traces of glue or wax.
– Sixth step:
Admire the result.