Why Polish, Symmetry, and Proportions Move Diamonds Up the Quality Grade

Only 0.4% of the 8.7 million diamonds in our scored index earn an A+ Caratlytics grade. The biggest reason most stones fall short isn't carat weight, colour, or clarity. It's Quality, the single heaviest component in the scoring system, and most of its power sits in details that never make it onto the showroom card.

We built Caratlytics to give every diamond a single comparable score from 0 to 100. The full methodology breakdown explains how five subscores fit together: Quality, Value, Certification, Market, and Presentation. Quality alone carries 35% of the total. This post covers what lives inside that 35%, and why polish and symmetry shift scores more than most buyers would guess.

What actually goes into a Quality score

The 4Cs matter. Cut grade, colour, clarity, and carat weight all feed into Quality. But they're one input group among many, not the whole story.

The Quality subscore evaluates eleven distinct inputs:

• Cut grade (where one exists on the certificate)
• Colour grade
• Clarity grade
• Polish grade
• Symmetry grade
• Fluorescence intensity
• Girdle thickness
• Culet size
• Table percentage
• Depth percentage
• Length to width ratio (for fancy shapes)

Buyers who stop at "the 4Cs" are ignoring more than half of what determines Quality. A stone with F colour, VS1 clarity, and Excellent cut can still land a middling subscore if its polish sits at Very Good, its symmetry at Good, its table runs wide, and it carries Strong fluorescence. Each of those details feeds the algorithm independently, and they add up fast.

We weight these inputs based on how much they actually affect a diamond's visual performance and durability. Polish and symmetry carry considerably more weight than most grading shorthand would suggest. So do proportions, particularly for fancy shapes where no lab issued cut grade exists.

Polish and symmetry punch above their weight

Most retailers treat polish and symmetry as afterthoughts. "Triple Excellent" earns a mention in the listing title, then the conversation shifts to colour and clarity. Inside the Quality subscore, though, polish and symmetry grades create real separation between otherwise similar stones.

GIA grades both on the same scale: Excellent, Very Good, Good, Fair, Poor. For rounds, a diamond can also receive an Ideal designation from certain labs (AGS uses its own system, but the principle holds). The jump from Very Good to Excellent on both polish and symmetry doesn't just tick a box. It reflects measurable differences in light return and visual crispness that show up under standard viewing conditions.

Across our index, stones with both polish and symmetry graded Excellent cluster in the upper scoring tiers. Drop one or both to Very Good, and the Quality subscore shifts downward by a margin that often surprises buyers shopping on the 4Cs alone. The effect compounds: a round brilliant with Excellent cut but Very Good polish and Very Good symmetry won't score the same as its triple Excellent counterpart, even when every other spec matches perfectly. That gap is real, and it's consistent across the 8.7 million stones we score.

Caratlytics Grade	Stones	Share of Index
A+ (90 to 100)	32,263	0.4%
A (80 to 89)	3,253,800	37.4%
B (70 to 79)	2,299,009	26.4%
C (60 to 69)	1,952,430	22.4%
D (below 60)	1,171,391	13.5%

Look at the A+ tier: 32,263 stones out of 8.7 million. Getting there requires strong marks across all eleven Quality inputs. Excellent polish and symmetry aren't sufficient on their own, but they're close to necessary. Meanwhile, nearly 36% of the index sits in C or D territory, and weak polish or symmetry grades are a recurring theme in those bands.

Fluorescence, without the drama

Fluorescence generates more confusion than almost any other diamond attribute. We treat it simply.

Strong or Very Strong fluorescence gets penalised in the Quality subscore. These levels can cause a milky or hazy appearance under certain lighting, and market pricing already reflects a discount for them. Our scoring follows the market reality.

Faint fluorescence is treated as neutral. It has no meaningful visual impact in the vast majority of stones, and penalising it would punish diamonds that look perfectly fine to any observer.

Medium fluorescence sits in between. We apply a small penalty, proportional to what the market data shows in terms of price impact. Not a cliff, not zero. A gentle slope that matches the way buyers actually trade these stones.

For lab-grown diamonds, we ignore fluorescence entirely. Lab-grown stones can exhibit fluorescence, but the market doesn't price it the same way, and the visual effects are less consistent across growers. Scoring it identically to natural fluorescence would misrepresent the actual quality picture. Average Quality scores bear this out: naturals average 71.9 across the index, while lab-grown stones average 74.2. Part of that gap comes from the fluorescence treatment, though higher production consistency in lab-grown also plays a role.

This is a good example of why a scoring system needs to reflect market behaviour rather than theoretical gemology. Fluorescence isn't inherently bad. But the data shows that Strong blue fluorescence in a high colour natural diamond correlates with lower realised prices, and our Quality subscore respects that.

The cut grade gap on fancy shapes

GIA issues a cut grade for round brilliant diamonds. For ovals, pears, marquises, cushions, radiants, emeralds, and everything else: no cut grade on the certificate.

Some scoring systems try to fill that gap by inventing their own cut grades for fancy shapes. We don't. Fabricating a single cut grade for an oval or a pear requires making aesthetic judgements that are genuinely subjective. What length to width ratio makes a pear look right? Opinions differ, and they should. Some buyers prefer elongated ovals; others want something rounder. No single correct answer exists, and pretending otherwise would inject false precision into the score.

Instead, we evaluate fancy shapes on the measurable proportions that do affect light performance: table percentage, depth percentage, and length to width ratio. These feed into Quality alongside polish, symmetry, and the other inputs. A pear with a 56% table and 62% depth scores differently from one at 64% table and 68% depth, because those proportions produce genuinely different optical results. Neither set of proportions is "wrong," but they are different, and the subscore captures that.

For rounds, GIA's cut grade is reliable and thoroughly calibrated, so we use it directly. For everything else, the individual proportion measurements carry the analytical weight. Fancy shape Quality scores end up driven more heavily by polish, symmetry, and proportions than round scores, since the lab's own cut grade handles much of the work on rounds.

A worked example that makes the gap concrete

Consider two 1.50ct G VS2 round brilliants, both GIA certified, priced within a few hundred dollars of each other.

Stone A: Excellent cut, Excellent polish, Excellent symmetry. Table at 57%, depth at 61.5%. No fluorescence. Girdle runs thin to slightly thick. No culet.

Stone B: Excellent cut, Very Good polish, Very Good symmetry. Table at 60%, depth at 63%. Medium fluorescence. Girdle runs slightly thick to thick. No culet.

Same carat weight. Same colour. Same clarity. Same cut grade. On a typical product listing page, they'd look like near twins.

They aren't. Not close.

Attribute	Stone A	Stone B
Cut	Excellent	Excellent
Polish	Excellent	Very Good
Symmetry	Excellent	Very Good
Table	57%	60%
Depth	61.5%	63%
Fluorescence	None	Medium
Girdle	Thin to Slightly Thick	Slightly Thick to Thick

Stone A hits strong marks across every Quality input. Tight proportions, top polish and symmetry grades, no fluorescence, a sensible girdle range. Stone B loses ground in five places simultaneously: polish, symmetry, table width, depth, and fluorescence. Each individual downgrade is small. Together, they compound into a meaningful Quality subscore gap that can push a stone from the A band down toward the B band, even though the headline specs look identical.

If these two stones sit within $200 of each other, Stone A is the clear better buy. If Stone B carries a significant discount, that discount might justify the Quality difference. That tradeoff is exactly where the Value subscore picks up the analysis, weighing what you get against what you pay. Quality tells you the diamond's grade. Value tells you whether the price matches.

Why this changes how you should search

Most diamond search filters let you set cut, colour, and clarity. Fewer let you filter by polish and symmetry specifically. Almost none let you filter by table percentage, depth percentage, or girdle thickness.

That's a problem, because those are precisely the attributes that separate a stone scoring in the A range from one sitting in the B or C band. Among our 8.7 million scored stones, 37.4% land in the A band and 26.4% in the B band. The gap between those tiers is often not the headline specs. It's the detail specs that get buried three clicks deep in a product listing.

Sorting by Caratlytics score means all eleven Quality inputs are already factored into the ranking. You don't need to manually compare polish, symmetry, table, and depth on every stone you consider. The score does it.

Quality is 35% of the total, but it doesn't act alone. The Value subscore assesses price against comparable stones across 110+ retailers, and the Certification subscore explains why a GIA graded stone and an IGI graded stone with identical specs don't start from the same baseline. Between those three components, you're looking at the vast majority of what separates the strongest deals in the market from the rest. And with 0.4% of stones reaching A+, the bar is genuinely high.