The 5th C: How Carat Hunter Scores 16 Million Diamonds

Only 0.4% of the 8.7 million diamonds we score earn an A+ grade. That single statistic tells you more about the market than any colour or clarity chart ever will. The Caratlytics Score is our proprietary fifth C: a composite rating that synthesises quality, value, certification integrity, and market positioning into one number between 0 and 100. We built it because the traditional 4Cs, for all their dominance in consumer education, were never designed to answer the question buyers actually have. Is this diamond good value for money?

This is the canonical explanation of how our diamond scoring methodology works, what feeds into it, and why we weight each component the way we do. If you've ever wondered why two diamonds with identical specs can land 30 points apart on our automated diamond grading system, this is where that logic lives.

Where the 4Cs fail you

The 4Cs are a grading language. They describe a diamond. They do not evaluate it.

A 1.50ct G VS2 Round with Excellent cut sounds like a specification you can shop confidently on. But that spec tells you nothing about whether the stone is priced fairly relative to its peers. It tells you nothing about whether the GIA report it carries is more or less reliable than an IGI or GCAL certificate for that particular combination of characteristics. And it tells you nothing about how this stone compares to the 4,000 other diamonds on the market with the same spec sheet.

This is the structural gap. The 4Cs give you description without context. A buyer comparing two G VS2 rounds at different retailers has no framework for deciding which represents better buying. One might be $5,200 and the other $6,800, but without understanding fluorescence, table percentages, crown angles, symmetry deductions, and the credibility of each grading lab for that specific grade combination, the price gap is just noise.

Three specific failures define the problem:

No value signal. The 4Cs don't reference price at all. Two identical specs at wildly different prices look the same on paper. A buyer has no way to know if $6,800 is competitive or extortionate for that particular stone in today's market.

No certification confidence. A G colour graded by GIA and a G colour graded by a lesser lab are not the same thing. Everyone in the trade knows this. Consumers mostly don't. The 4Cs treat all certificates as equal, which they categorically are not.

No market context. Is demand for 1.5ct G VS2 rounds rising or falling? Is this retailer historically expensive? Is there oversupply in this spec category that should push prices down? The 4Cs are static. Markets are not.

We've tracked these gaps across more than 16 million diamonds from 110+ retailers. The variance within any single 4C combination is enormous. Two stones graded identically can differ in real optical performance, certification reliability, and fair market value by a factor that makes the 4Cs alone almost useless as a buying tool. The Caratlytics Score, our 5th C diamond rating, exists to close that gap.

Four scores, one number

Every scored diamond receives four subscores, each weighted according to its importance in the overall buying decision.

Quality: 35%

The single largest component. This isn't just the 4C grades restated; it's our assessment of how well the stone performs within its stated grade. A VS2 with a table inclusion in a position that affects light return is not the same as a VS2 with a feather tucked under a bezel facet. We analyse cut proportions, symmetry deviations, fluorescence interactions, and inclusion positioning to determine where a stone sits on the quality spectrum within its nominal grade. Our full quality methodology breaks down every input.

Value: 30%

How does this stone's price compare to similarly specified diamonds in our index right now? Not last month, not a historical average. Today. We compute a fair market range for every combination of specs and measure where each stone falls relative to that range. A diamond priced at the 15th percentile of its peer group scores very differently from one at the 85th percentile. The value score methodology covers the full calculation.

Certification: 20%

Not all grading labs are equal, and not all grades from the same lab carry equal confidence. We've built internal consistency models that flag certificates where the stated grade is statistically unlikely given the stone's other characteristics. A diamond with a colour grade that sits two standard deviations above what its fluorescence and clarity combination would predict gets a certification discount. Full certification scoring here.

Market: 15%

Where is this stone positioned within the broader market? Is demand for this spec category rising or falling? Has the retailer holding this stone historically priced competitively? The market component captures dynamics that the other three scores treat as static. It rewards stones that sit in competitively priced retail channels during favourable demand conditions.

How the grades distribute

Raw scores translate to letter grades on a scale that mirrors academic marking. Deliberately.

Grade	Score range	What it means
A+	90 to 100	Exceptional across all four dimensions. Rare.
A	80 to 89	Strong performer. Solid quality, fairly priced, reliably certified.
B	70 to 79	Acceptable. Minor weaknesses in one or two subscores.
C	60 to 69	Below average. Notable gaps in value or certification confidence.
D	Below 60	Poor overall proposition. Significant issues in multiple dimensions.

Here's how the current live index distributes across those bands:

Grade band	Stones	% 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%

Total scored stones: 8,708,893 across both natural and lab-grown inventory.

The distribution is intentionally skewed to the right. We calibrated the thresholds so that A+ remains genuinely elite. If 10% of diamonds scored A+, the grade would mean nothing. At 0.4%, it means something. Meanwhile, 37.4% earning an A tells you that good diamonds aren't actually scarce. The market has plenty of fairly priced, reliably certified, quality stones. The challenge is finding them among the 36% that score C or D.

One detail that often surprises people: lab-grown diamonds average slightly higher (74.2) than naturals (71.9). This isn't because lab-grown stones are inherently better. It reflects their market positioning. Lab-grown stones tend to score well on value (they're priced aggressively) and on certification (major labs have more consistent grading for lab-grown). They lose ground on market dynamics where resale uncertainty pulls them down.

What an A+ actually looks like

A 1.52ct E VS1 Round, GIA certified, listed at $9,450.

Quality subscore: 94/100

Table 57%, crown angle 34.8 degrees, pavilion angle 40.8 degrees. These proportions land in the centre of GIA Excellent and also in what independent optical analysis confirms as peak light return territory. No fluorescence. The VS1 inclusion is a minor pinpoint cluster positioned near the girdle, functionally irrelevant to visual performance. Symmetry graded as Excellent with no measurable deviations in our analysis. This is about as good as a round brilliant gets within its stated grades.

Value subscore: 88/100

At $9,450, this stone sits at roughly the 22nd percentile of pricing for 1.5 to 1.6ct E VS1 Rounds with Excellent cut in our index. Not the cheapest available, but well below the median and significantly below the mean. The stones priced lower tend to have fluorescence or less optimal proportions, which would drag their quality scores. Genuinely good buying for what it is.

Certification subscore: 91/100

GIA carries the highest baseline confidence in our model. For E colour specifically, GIA's consistency record is strong. The VS1 grade aligns with what we'd expect given the inclusion map. No red flags. Full marks minus a small standard deduction we apply to all certificates (because no lab is infallible).

Market subscore: 89/100

The retailer has a historical pricing pattern that trends competitive. Demand for 1.5ct E VS1 Rounds is stable with slight upward pressure. No oversupply signals in this spec category. Solid market position.

Composite: 91.3 (A+)

Weighted calculation: (94 x 0.35) + (88 x 0.30) + (91 x 0.20) + (89 x 0.15) = 32.9 + 26.4 + 18.2 + 13.35 = 90.85, rounds to 91. This stone earns its A+ by performing consistently across all four dimensions. No single score carries the others. That's what separates A+ from A: balance.

What a C looks like

A 1.50ct G VS2 Round, IGI certified, listed at $7,200.

Quality subscore: 68/100

Cut grade is listed as Excellent, but the proportions tell a different story. Table 62%, pavilion angle 41.4 degrees. That pavilion depth creates light leakage visible in any ASET analysis. The stone will face up slightly dark. Add medium blue fluorescence, which at G colour can create a milky appearance under certain lighting. The VS2 inclusion is a crystal positioned directly under the table. Eye-clean under most conditions, but just barely. This is a stone that technically hits its 4C grades but underperforms within them.

Value subscore: 52/100

$7,200 for a 1.5ct G VS2 Round with these characteristics is expensive. The median for this spec category sits around $6,400 in our index. Stones with better proportions and no fluorescence are available for less money. This diamond is priced as though it were a top quartile example of its grade combination, and it simply isn't.

Certification subscore: 61/100

IGI carries a lower baseline confidence than GIA in our model for natural stones in this grade range. Specifically for G colour with medium fluorescence, we've observed that IGI reports tend to be generous relative to GIA assessments. The stated VS2 is plausible but sits at the boundary where lab to lab variation is highest. We apply a confidence haircut that reflects this statistical pattern.

Market subscore: 70/100

The retailer prices consistently at or above market median. Demand for 1.5ct G VS2 is healthy, which prevents the market score from dropping further. But the retail positioning is not doing this stone any favours.

Composite: 62.1 (C)

Weighted: (68 x 0.35) + (52 x 0.30) + (61 x 0.20) + (70 x 0.15) = 23.8 + 15.6 + 12.2 + 10.5 = 62.1. A C grade. Not the worst diamond in the index, but one that a buyer could easily improve upon by spending the same money (or less) on a higher scoring alternative in the same spec range.

The gap between these two stones is 29 points. Both are roughly 1.5ct rounds in the E to G, VS1 to VS2 space. The 4Cs alone would not have surfaced how different they are as buying propositions.

What feeds each subscore

Subscore	Weight	Primary inputs	Data sources
Quality	35%	Cut proportions, symmetry, fluorescence, inclusion type and position, optical performance indicators	GIA, IGI, GCAL reports; proportion data; inclusion maps
Value	30%	Price relative to peer group, percentile ranking, historical price trajectory for spec category	Live pricing from 110+ retailers; Carat Hunter price index
Certification	20%	Lab identity, lab consistency for specific grade combo, statistical likelihood of stated grades, cross-retailer duplicate detection	Internal grading consistency models; cross-retailer pattern analysis
Market	15%	Retailer pricing history, demand signals for spec category, supply concentration, resale market indicators	Retailer tracking data; market demand indices; resale platform monitoring

Every input is derived from structured certificate data or observed market behaviour. No human editorial judgement enters the scoring. We tune the models quarterly, but the individual diamond scores are fully algorithmic. That's non-negotiable. The moment a scoring system includes subjective overrides is the moment it becomes a marketing tool instead of a measurement tool.

The questions we get asked

Why not weight value higher? Isn't price the whole point?

Price matters enormously, but a cheap diamond with a dubious certificate or poor optical performance isn't a bargain. It's a trap. We weight quality highest because a diamond that performs beautifully within its grade, carries a reliable certificate, and is priced fairly will always be a better purchase than one that's merely cheap. Value at 30% still gives it massive influence on the final score. A genuinely underpriced stone gains nearly a full letter grade from value alone.

How do you handle lab-grown differently from natural?

The scoring framework is identical. The peer groups are separate. A lab-grown G VS2 is compared to other lab-grown G VS2s for its value score, not to natural equivalents. Quality metrics, certification models, and market signals are all calibrated within origin. This is why lab-grown averages 74.2 versus natural at 71.9: the comparison is always like for like.

Can a diamond with a low colour or clarity still score A+?

Yes. We've seen J SI1 stones score A+ because they had exceptional cut quality, were priced aggressively, carried a highly consistent GIA report, and sat in a favourable market segment. The caratlytics score measures how good a diamond is as a purchase, not how rare or prestigious its specs are on paper. A J SI1 that nails every dimension is a better buy than an F VVS2 that's overpriced and questionably graded.

Why does certification get 20%? That seems high.

Because we've found that grading inconsistency is one of the largest hidden costs in diamond buying. A stone graded G by one lab might have been graded H or even I by a stricter lab. That one grade difference can mean $1,000 or more in price premium that the buyer is paying for a letter on paper, not a genuine characteristic of the stone. Twenty percent reflects the real financial impact of certification uncertainty.

Do you penalise specific retailers?

We don't apply manual penalties to any retailer. The market subscore reflects observed pricing patterns and competitive positioning as measured by our algorithms. If a retailer consistently prices above market for equivalent stones, that shows up in the score. No editorial judgement involved.

Is the score static or does it change?

It changes. A diamond's quality subscore is relatively stable (proportions don't change), but its value, certification, and market subscores update as pricing moves, as our lab consistency models refine, and as demand shifts. A stone that scored B last month could score A today if its price dropped or if competing inventory thinned out. We recalculate the full index continuously.

What comes next

This piece is the framework. The real depth lives in the four dedicated methodology articles that break each subscore into its full technical detail: how we assess quality, how value scoring works, and how certification confidence is calculated. A fourth piece on market scoring is coming shortly. Each of those articles includes the specific algorithms, the edge cases we've tuned for, and the data that informed our weighting decisions. If you're the kind of buyer who wants to understand not just what the score means but exactly how it's computed, those are your next reads.

And if you just want to use the score to filter diamonds? Open our advanced search, set the minimum grade to A, and let 36% of the market fall away.