Carving a gem according to the rules of the art is the lapidary's ability to bring out the maximum hidden potential of the stone. A poorly executed cut will not render as well as one made with full attention.

A cut approaching the ideal involves several determining factors that vary from one gem to another.

The art of enhancing gems also depends on the quality of the starting stone. A medium-quality stone cannot be transformed into an exceptional one.

In the case of recutting, one should not confuse a low-quality stone with a poor cut. Note that an experienced lapidary will be able to give you an opinion on the potential of your gem.

Although it is possible to improvise a cut, the lapidary may choose to design a cutting plan before starting the project.

The choice of diagram will impact the gem's appearance. Also note that the same diagram cut on gems of different varieties will not render the same way: the refractive index changes, and the light wave will not propagate the same way through the gem.

In many cases, the cutter ensures that the diagram angles do not create a window (see diagram below). Without going into technical details, a window corresponds to light rays that 'escape' through the back of the gem without returning to the observer.

In practice, if a stone has a window: you will be able to read through it if you place it over a newspaper while looking at it from the front. This will not be the case in a gem without a window: total internal reflection (TIR) respected.

Many so-called 'commercial' cuts include a window. Often, this allows the lapidary to round the pavilion and preserve weight at the expense of aesthetic criteria.

Explanatory diagram of light ray displacement through a gem :

This diagram is a summary of a first step towards a successful cut. It quickly explains the impact of angles on total internal reflection.

However, I find it a bit reductive because it is not that simple; there are other factors to consider. To name a few: gem symmetry, arrangement of facets relative to each other, quality of meeting points, polishing quality, flatness of facets, etc.

Technical note : you have probably seen very deep quartz that looked great once cut. Quartz has a low refractive index (gemology concept), resulting in a high critical angle. To avoid a window, the lapidary must add angle to the pavilion to stay above the critical angle. Note that some cutting plans accentuate this phenomenon more than others. As you understand, this diagram is a shortcut that omits many parameters and does not work in all cases depending on what the lapidary aims to achieve.

In transition with what I explained in the previous box, I also wanted to add that the cutting plan influences the color result after cutting. An experienced lapidary will consider this before starting the project.

Note that the starting color gives an idea of the result, not a certainty regarding the final project.

Gems with the most potential are those with what I call 'open color'—meaning they are not dark. Some say that a window should be cut to lighten a dark gem. If you do not like dark stones, I advise you not to buy this type of gem rather than hoping to improve them.

The placement of color in the future cut gem is an important criterion. Some stones are not homogeneous and show more or less saturated colors in the rough. In this case, when possible, the lapidary arranges the most saturated color in the center of the stone at the pavilion tip or higher in the gem. This creates the illusion that the gem viewed from the front is perfectly uniform in color.

Examples of non-homogeneous color distribution in a cut stone:

Stones are sometimes dichroic or trichroic (gemology concept): different colors depending on the viewing direction. In this case, the lapidary will place the table parallel to the most desirable color. The pavilion shape (elongated, pointed, etc.) will also play a role in blocking or allowing underlying color to pass. Tourmalines are a perfect example of dichroism. They can be green along the growth axis and yellow perpendicular to it, or vice versa.

I wanted to mention a specific case where one axis of the gem is dark and the other is not. You have probably seen rectangular verdelite or indicolite with cut corners and pavilion sides almost parallel to the foliation. Often, the growth axis is fairly dark and the lapidary had no choice but to cut it this way. Otherwise, the gem would have been very dark.

Diagram showing elongated pavilion to block undesired color along the c-axis:

There are also bicolor gems. The lapidary will prefer a slightly elongated pavilion to separate colors. A pavilion ending in a point would tend to mix colors. I have cut some ametrines mixing colors, and the result can be just as spectacular.

Inclusions can sometimes be hidden with a nice cut, or conversely, they can hinder brilliance if too numerous. Most lapidaries prefer relatively 'clean' gems for faceting.

Indeed, an inclusion can sometimes extend during the cutting process due to internal stresses released when placing the gem on the wheels. Often, this is the start of a fracture or cleavage.

Regarding cleavages and the art of cutting, see another article on this topic -> Cleavage plane and cutting: how to proceed?

In the case of a very included stone, this adds an element of randomness to the result after cutting: inclusions deviate light rays in unexpected directions. A beautiful cut takes time, so maximize chances by selecting the best materials.

However, not all stones are free of inclusions. It would be a shame not to cut certain roughs just because they have a few small inclusions. For example, it is possible to place an inclusion along the foliation so it does not hinder brilliance, or perpendicular to the table axis. In both cases, they will be almost invisible, and the stone will still be radiant with potential for jewelry.

As you can see, there are many scenarios and multiple solutions. Creating an original cut for a gem is an art guided by the lapidary's choices. However, the lapidary must first adapt to the material at hand. Each stone is different and presents its own challenges. An experienced lapidary observes, thinks, and then creates.

This article covers only a small part of the art of cutting; much more could be added.

The goal was to explain broadly, in a pedagogical and accessible way, some of the factors involved in achieving a successful cut.

You understand that the lapidary practices a craft in which he creates at will. As in all forms of art, this can generate misunderstandings or questions but also bring joy with the stones he works along the way.

Thank you for your kind attention to this article; I greatly enjoyed writing it.