When buying a diamond, most people focus on the famous Four Cs: Carat, Colour, Clarity, and Cut. However, if you’ve looked closely at a GIA grading report, you’ve probably noticed two additional categories listed beneath the cut grade:
- Polish
- Symmetry
For many buyers, these terms are confusing. They sound similar, and because they are usually listed together, it’s easy to assume they describe the same thing.
In reality, they measure two completely different aspects of a diamond’s craftsmanship.
Both polish and symmetry contribute to the overall quality of a finished diamond, but neither should be confused with the cut grade itself. A diamond can have Excellent Polish but only Very Good Symmetry, or vice versa. More importantly, a diamond can even have Excellent Polish and Excellent Symmetry yet still perform poorly if its proportions are not well designed.
Understanding the difference between polish and symmetry will help you interpret a grading report more accurately and avoid paying for features that may have little impact on what you actually see.
What Is Diamond Polish?
After a diamond has been cut into its final shape, each facet must be polished until it becomes exceptionally smooth.
Polish refers to the quality of a diamond’s surface finish.
Think of polishing as the final refinement stage. Even if the facets are cut to perfect angles, microscopic imperfections left on the surface can slightly affect the way light reflects.
Professional polishers use specialised equipment coated with diamond powder—the hardest abrasive known—to create an almost mirror-like finish on every facet.
The smoother the surface, the more efficiently light can enter and leave the diamond.
What Does GIA Evaluate for Polish?
When grading polish, laboratories examine the surface of the diamond under high magnification.
They look for microscopic imperfections such as:
- Fine polishing lines
- Burn marks
- Abrasions
- Surface pits
- Tiny scratches
- Wheel marks
- Lizard skin (a subtle textured appearance)
- Surface grain lines
Most of these imperfections are completely invisible to the naked eye.
Only under magnification can trained gemologists identify them.
GIA Polish Grades
The Gemological Institute of America assigns one of five polish grades:
- Excellent
- Very Good
- Good
- Fair
- Poor
For modern jewellery-quality diamonds, Excellent and Very Good are by far the most common.
Does Polish Affect Sparkle?
Yes—but usually only slightly.
Unless the polish is unusually poor, most buyers will never notice a visible difference between:
- Excellent Polish
- Very Good Polish
The surface imperfections separating these grades are often microscopic.
In everyday viewing conditions, they have little effect on the diamond’s brilliance.
This surprises many buyers, who assume polish is one of the most important grading factors.
In reality, proportions and cut quality have a much greater influence on sparkle.
What Is Diamond Symmetry?
Symmetry is completely different.
Instead of evaluating the smoothness of the facets, symmetry measures how accurately every facet has been positioned and aligned.
Imagine drawing a perfectly symmetrical snowflake.
Every section should mirror the others.
A Round Brilliant diamond follows the same principle.
Each facet should be:
- Correctly shaped
- Correctly positioned
- Properly aligned
- Uniform in size
The more precise the alignment, the more balanced the diamond’s optical performance becomes.
What Does GIA Evaluate for Symmetry?
When assessing symmetry, laboratories examine whether:
- Facets meet at the correct points.
- Opposing facets are identical.
- The table is centred.
- The culet is centred.
- Facets are evenly shaped.
- Girdle thickness is consistent.
- The overall outline is circular.
Small misalignments can slightly alter the path of light inside the diamond.
GIA Symmetry Grades
Like polish, symmetry receives one of five grades:
- Excellent
- Very Good
- Good
- Fair
- Poor
Again, Excellent and Very Good are the grades most commonly found in quality jewellery.
How Symmetry Affects Light Performance
Unlike polish, symmetry has a more direct relationship with light behaviour.
When facets are positioned precisely:
- Light reflects consistently.
- Contrast patterns become sharper.
- Scintillation becomes more balanced.
- Brightness appears evenly distributed.
Poor symmetry can slightly reduce optical precision because some light no longer follows the intended path.
However, it’s important to understand that symmetry alone does not determine brilliance.
Can You See the Difference?
For most buyers:
Excellent vs Very Good Polish
Almost impossible to distinguish without magnification.
Excellent vs Very Good Symmetry
Usually very difficult to distinguish without comparing diamonds side by side.
In most cases, differences become visible only under specialised lighting or through precision optical tools.
This is why experienced jewellers often recommend prioritising cut proportions before insisting on Excellent polish or symmetry.
Polish and Symmetry Are Not the Same as Cut
One of the most common misconceptions is believing that Excellent Polish and Excellent Symmetry automatically produce an exceptional diamond.
This is not true.
Consider these two diamonds.
Diamond A
- Excellent Cut
- Excellent Polish
- Excellent Symmetry
Diamond B
- Excellent Polish
- Excellent Symmetry
- Poor overall proportions
Although Diamond B has excellent craftsmanship, its proportions may still cause significant light leakage.
The diamond may appear less brilliant than Diamond A.
Good craftsmanship cannot compensate for poor design.
An Easy Analogy
Imagine building two luxury sports cars.
The first has:
- Perfect paintwork
- Perfect panel alignment
- Excellent engine design
The second has:
- Perfect paintwork
- Perfect panel alignment
- A poorly designed engine
Both cars look beautifully finished.
However, only one performs exceptionally.
In diamonds:
- Cut proportions are the engine.
- Polish is the paint finish.
- Symmetry is the body alignment.
All three matter, but the underlying design has the greatest influence on performance.
Why Hearts and Arrows Diamonds Require Excellent Symmetry
One area where symmetry becomes extremely important is in Hearts and Arrows diamonds.
These diamonds display perfectly formed heart and arrow patterns when viewed through specialised scopes.
This optical precision requires:
- Exceptional facet alignment
- Precise indexing
- Outstanding craftsmanship
Even tiny symmetry deviations can distort these patterns.
Not every Excellent Symmetry diamond qualifies as Hearts and Arrows, but every genuine Hearts and Arrows diamond demonstrates extraordinary symmetry.
When Is Excellent Polish Worth Paying For?
Excellent Polish is usually worthwhile because the price difference is often modest.
However, if choosing between:
- Excellent Polish
- Very Good Polish
most professionals would prioritise:
- Better cut proportions
- Better pavilion angle
- Better crown angle
before insisting on Excellent Polish.
The visual improvement from better proportions is significantly greater.
When Is Excellent Symmetry Worth Paying For?
Excellent Symmetry becomes more valuable when:
- Purchasing a Round Brilliant diamond.
- Seeking maximum optical precision.
- Buying a Hearts and Arrows diamond.
- Investing in premium craftsmanship.
For everyday buyers, Very Good Symmetry can still produce an exceptionally beautiful diamond if the overall proportions are well balanced.
Practical Buying Advice
For most buyers, the following combination offers an excellent balance between beauty and value:
- Excellent Cut
- Excellent Polish
- Very Good or Excellent Symmetry
If your budget allows, choosing Excellent in all three categories provides additional confidence in craftsmanship.
However, never sacrifice ideal proportions simply to obtain higher polish or symmetry grades.
Common Buying Mistakes
Believing Polish Creates Sparkle
Sparkle comes primarily from light performance, not surface smoothness.
Polish refines sparkle rather than creating it.
Assuming Excellent Symmetry Guarantees Maximum Brilliance
Symmetry improves optical precision but cannot overcome poor crown or pavilion angles.
Paying Large Premiums for Microscopic Differences
Many polish differences are visible only under magnification.
Before paying significantly more, ensure the improvement produces a meaningful visual benefit.
What Do Professional Buyers Prioritise?
Many experienced diamond buyers evaluate characteristics in roughly this order:
- Light performance
- Overall cut proportions
- Pavilion angle
- Crown angle
- Table percentage
- Depth percentage
- Symmetry
- Polish
- Colour
- Clarity
This order may surprise first-time buyers, but it reflects the reality that sparkle attracts attention long before microscopic finishing details.
Final Thoughts
Polish and symmetry are both important indicators of craftsmanship, but they serve different purposes.
Polish measures how smoothly each facet has been finished, while symmetry measures how accurately those facets have been aligned. Together, they contribute to a diamond’s overall quality and help refine its optical performance.
However, they should never be viewed in isolation. A beautifully polished and perfectly symmetrical diamond cannot compensate for poor proportions or inefficient light return.
When choosing a diamond, think of polish and symmetry as the finishing touches on an already well-designed masterpiece. Once you’ve found a diamond with excellent proportions and strong light performance, these characteristics provide additional assurance that the stone has been crafted with exceptional care.
Key Takeaways
- Polish measures the smoothness and quality of each facet’s surface.
- Symmetry measures how accurately the facets are aligned and positioned.
- Both receive grades ranging from Poor to Excellent.
- Excellent Polish and Excellent Symmetry reflect superior craftsmanship but do not guarantee superior sparkle.
- Cut proportions have a much greater influence on brilliance than polish or symmetry alone.
- Very Good Polish or Symmetry is often visually indistinguishable from Excellent to the naked eye.
- Prioritise overall light performance and proportions before paying a significant premium for microscopic finishing improvements.