How Diamonds Are Cut (Part 2): Polishing and Symmetry Explained

In the first part of this series, we explored how a rough diamond is studied, mapped, and planned long before any cutting begins. That stage is where the future of the diamond is essentially decided, from its final shape to how much it will weigh. Once that blueprint is complete, the process shifts from planning to execution, and this is where the diamond starts to physically take form. But even at this stage, it is still far from finished. What exists at this point is a shaped but unfinished stone that holds potential that’s brought to life through polishing and symmetry.

These final stages are where a diamond begins to show the optical performance people associate with fine jewelry. The sparkle, brightness, and fiery flashes of light are the result of extremely precise finishing work. This is why polishing and symmetry are considered just as important as the earlier stages, even though they happen later in the process.

The Transition from Shaping to Polishing

After a diamond has been shaped into its basic form, it begins to resemble something closer to a finished gem, but the surface is still far from refined. At this stage, the facets may be roughly visible, but they’ll look dull and won’t properly disperse light. Polishing is the stage where those rough surfaces are gradually refined until they are smooth and reflective. It is a slow, deliberate process where every facet is carefully worked on until it meets exact specifications. Unlike shaping, which focuses on creating the basic geometry of the diamond, polishing is about refinement at a microscopic level. Even the smallest imperfections can influence how light passes through the stone.

What surprises many people is that a freshly cut diamond does not naturally sparkle. Without polishing, even a perfectly proportioned stone can appear cloudy or lifeless. The brilliance associated with diamonds only appears once the surfaces have been carefully finished to the point where they can interact with light efficiently. 

Why Diamond Polishing Is So Challenging

Polishing a diamond is one of the most technically demanding steps in the entire cutting process, mostly because of the material. Diamonds are the hardest natural substance known, which makes them incredibly resistant to wear, but also extremely difficult to shape with precision. Every adjustment requires patience, control, and a deep understanding of how the material responds under pressure.

One of the most interesting aspects of polishing is that diamonds are essentially polished using other diamonds. The wheels and surfaces used in this process are coated with diamond powder, which allows the cutter to gradually refine each facet. This creates a situation where the hardest natural material on Earth is both the subject and the tool of the work, adding another layer of complexity to the process.

Each movement during polishing is carefully controlled because the removal of material is permanent. Even microscopic changes can change proportions or affect how light travels through the stone. If too much is removed, the diamond may lose weight, but if too little is removed, the facet may remain imperfect, which can impact brilliance. This constant need for precision is what makes polishing such a specialized skill.

Understanding Facets

To understand why polishing matters so much, it helps to understand what facets actually do. Facets are the flat, intentionally cut surfaces on a diamond that control how light moves through the stone. They function like a network of mirrors to capture, redirect, and reflect light.

In a standard round brilliant diamond, there are typically 57 or 58 facets depending on the presence of a culet. Each one plays a role in shaping the diamond’s overall optical performance. Some facets are designed to allow light to enter the stone, while others reflect it internally, and others help disperse that light into the flashes of color often referred to as fire.

For this system to work effectively, each facet must be polished to a high level of smoothness. If the surface is uneven or contains microscopic scratches, it can interfere with how light is reflected. Instead of clean, sharp brilliance, the result may be a softer or less vibrant appearance.

What Diamond Polish Grades Mean

Because polishing has such a direct impact on appearance, it is carefully evaluated by grading laboratories as part of the overall cut assessment. These polish grades help define how well the surface of each facet has been finished and whether any imperfections are present that could affect performance.

The grading scale typically ranges from Excellent to Poor, with several levels in between. An Excellent polish grade indicates that the facets are extremely smooth, with no visible imperfections under normal viewing conditions and only minimal characteristics detectable under magnification. 

A Very Good polish grade represents a diamond that may have slight surface characteristics, but these are generally subtle and not easily noticeable without close inspection. In most cases, diamonds in this category still perform very similarly to Excellent stones under normal viewing conditions. As the grades decrease further, imperfections such as polishing lines, abrasions, or minor surface irregularities become more noticeable and can begin to influence how light behaves within the diamond.

For most buyers, diamonds in the Excellent or Very Good range offer the strongest balance between visual performance and value, since the differences are often minimal to the naked eye but can still influence pricing.

The Importance of Surface Smoothness

Polishing is ultimately about controlling how light interacts with the surface of each facet. When light enters a diamond, it is not simply reflected once before returning to the eye. Instead, it travels through a complex internal path, bouncing between multiple facets before exiting the stone. Each of these interactions depends on how smooth and precise those surfaces are.

If a facet contains even minor imperfections, it can scatter light in unintended directions. This can soften the overall sparkle of the diamond. Surface smoothness ensures that light behaves as intended and allows the stone to maintain brightness, contrast, and fire.

What Is Symmetry and Why Does It Matter?

While polishing focuses on the quality of individual surfaces, symmetry is about how those surfaces are arranged in relation to each other. In a round brilliant diamond, symmetry includes factors such as whether the table is centered, whether opposing facets are evenly matched, and whether the crown and pavilion are properly aligned. A diamond with strong symmetry has facets that are expertly aligned and proportioned. When symmetry is poor, the diamond can appear slightly off, which affects light performance. Instead of reflecting cleanly, light may leak out of the sides or bottom of the stone instead of returning to the viewer’s eye. When facets are symmetrical, the sparkle feels more evenly distributed across the stone.

The Relationship Between Polish and Symmetry

Polish and symmetry are evaluated separately, but they function together in shaping the final appearance of a diamond. A stone with excellent polish but weak symmetry may still have improperly directed light. A diamond with strong symmetry but poor polish won’t have smooth enough surfaces to reflect light properly.

When both elements are strong, the result is a diamond that allows light to enter, travel, and exit the stone with minimal disruption, producing the brightness and fire that we all desire.

Final Thoughts

By the time a diamond reaches the polishing and symmetry stages, most of the major decisions about its shape and proportions have already been made. But these final steps are what allows each facet to work together as a unified system capable of dispersing light properly.

The brilliance, fire, and scintillation that make diamonds so captivating are the result of countless precise adjustments, skilled craftsmanship, and a deep understanding of how light interacts with the stone. Together with the planning, mapping, and shaping stages discussed earlier in this series, polishing and symmetry complete the journey from rough crystal to finished diamond. What begins as an unremarkable piece of rough material ultimately becomes a gorgeous gemstone.