How to Compare Diamonds on a Budget & Read the Price
Set a diamond search to the top of your budget, and the results fill with stones that are hard to tell apart on the listing. Several will share a color grade and a clarity grade and fall within a few hundredths of a carat of each other, and their prices will still differ. Part of that difference comes from the weight bands natural diamonds are priced in. Another part comes from details the certificate records below the grades, such as measurements, polish, symmetry, and fluorescence.
How a Diamond Price Is Built
Natural diamonds are priced per carat, at a rate set by weight band. On the trade’s reference list, published by Rapaport, the bands between 0.50 and 3.00 carats break at 0.70, 0.90, 1.00, 1.50, and 2.00, and a stone a few hundredths over 1 carat can cost up to about 20% more at retail than one a few hundredths under it.
Lab-grown listings can show no step at all. In one retailer’s September 2026 listings, lab-grown rounds of 1.00 to 1.02 carats in D to F color were priced at $400.50 per carat, below the per-carat price of 0.91- to 0.97-carat stones with matching grades.
A stone below a band edge gives up less size than its weight difference suggests. Face-up size follows a diamond’s area and weight follows its volume. At matching proportions, a 0.90-carat round is about 7% smaller face-up than a 1.00 and under 4% narrower. That comes to roughly 0.2mm on a round of about 6.4mm.
That assumes the stones are cut alike. A deep cut puts weight below the girdle, where it adds nothing to the face-up view, and a heavier diamond cut deep can look no larger than a lighter one cut well. To find the step for a given color and clarity in natural stones, run one search at 0.90 to 0.99 carat and a second at 1.00 to 1.09, then divide each price by its carat weight.
Where the 4Cs Give Room
A lower color or clarity grade costs less. Yellow and rose gold mask some of the warmth in near-colorless diamonds, graded G to J, so a lower color grade can go further in a warm metal than in white gold or platinum. Clarity grades already weigh an inclusion’s size, number, position, nature, and relief. The facet style changes how easily an inclusion is found. Emerald and Asscher cuts show inclusions more readily than brilliant cuts and generally call for a higher clarity grade. On a round, the cut grade describes how the stone returns light, and a lower cut grade means less brightness and sparkle in any metal or setting.
Two Stones With Matching Grades
The Report Below the Grades
- Measurements. Length, width, and depth in millimeters. Two stones of equal weight can differ in face-up size here.
- Depth percentage. Total depth as a share of the stone’s width. A higher figure usually means less face-up size for the weight.
- Polish and symmetry. Very Good is hard to tell apart from Excellent with the unaided eye. At Good, polish can dull the stone’s luster and symmetry can affect its overall look.
- Fluorescence. Graded from None to Very Strong, and discounted in some color and clarity ranges.
Face-Up Size Against Price
A 1-carat round cut deeper comes out narrower. By the trade’s standard weight formula, a 1-carat round cut to 60% depth is about 6.5mm across, and one cut to 62% is about 6.4mm, a gap of roughly 2% in face-up area.
For two stones of the same outline, multiplying length by width from the measurement line gives a figure that compares their face-up size. A round’s two numbers are its smallest and largest diameters. The outline’s own constant cancels out when both stones share it, so the result does not need converting to true area. Dividing each price by that figure gives a cost per square millimeter of face-up size for each stone.
What Fluorescence Does to Price
When two otherwise matching stones differ in fluorescence, part of the price gap can be the discount the trade applies, steepest in high-color, high-clarity stones and close to nil around I to K color. Check the video for a cloudy look to spot rare stones where strong fluorescence creates a visible haze.
Inclusions & the Setting’s Prongs
GOODSTONE’s diamond search shows a diamond video in each listing’s details, so two stones with matching grades can be watched back to back. The clarity grade is assigned to the loose stone. Before choosing between two stones, compare where each one’s edge inclusions fall with where the prongs of the chosen setting will be, since a prong can cover an inclusion near the girdle once the diamond is mounted.
Reading an IGI Report Against a GIA Report
A lab-grown diamond on an IGI report and a natural one on a GIA report are graded under different rules. GIA gives a natural oval or pear polish and symmetry grades and no overall cut grade, and IGI has issued cut grades from Excellent to Poor for nine fancy outlines since 2022, with a shape-specific check for the bow-tie. Lab-grown stones on GIA reports get only a tier, Premium or Standard.
A G color from one lab and a G color from another are two separate judgments, and no peer-reviewed study has measured how closely GIA and IGI agree on the same stones.
The two reports share measurements. Millimeter dimensions, depth, and table mean the same thing on either report. The length-times-width comparison works across the two reports on those figures alone. Where the grades differ by a step, the video shows how much of that step is visible in each stone.
A Worked Comparison Across the 1-Carat Line
Two GIA-graded rounds listed in September 2026 share G color, VS2 clarity, and an Excellent cut grade, and they fall on either side of the 1-carat line.
| Guide | 0.90 carat | 1.00 carat |
|---|---|---|
| Price | $2,338.88 | $3,426.30 |
| Measurements | 6.12 × 6.19 × 3.90mm | 6.29 × 6.35 × 4.03mm |
| Depth | 63.3% | 63.7% |
| Polish / Symmetry | Excellent / Very Good | Excellent / Excellent |
| Fluorescence | Faint | Faint |
The 1.00 costs $1,087.42 more, or 46.5%. Multiplying length by width gives 37.9 for the 0.90 and 39.9 for the 1.00, about 5.4% more face-up area and a little under the 7% that matching proportions would give. Averaged across the two diameters of each stone, the 1.00 is about 0.17mm wider. Divided into each price, the length-times-width figure comes to $61.74 for the 0.90 and $85.78 for the 1.00, so each unit of face-up size costs about 39% more in the heavier stone.
The 0.90’s Very Good symmetry is one grade below Excellent, a step GIA’s testing found observers could not reliably see. Both stones show faint fluorescence, a strength the trade does not discount.
A lab-grown pair listed the same month shows no step. They are IGI-graded D VS1 rounds with Ideal cut grades, listed at 0.96 carats for $420.53 and 1.01 carats for $404.51. The heavier stone costs $16.02 less, and its length-times-width figure is 41.6 against 40.8, about 2% more face-up area.
Before choosing, look up each report number with the lab that issued it.