This index tracks listed retail prices (not transaction prices) across major UK online lab-grown diamond retailers. The September 2026 edition collected 2,213 round G VS1 IGI lab-grown diamond listings (before each retailer's own Excellent-cut filter) across three retailers. All prices are in GBP, inclusive of 20% UK VAT (inc-VAT). Data was collected on September 17, 2026. This is the fifth edition; the retailer panel is unchanged since the May 2026 inaugural edition.

Quality Diamonds' listed inventory contracted sharply between the August and September collections: 74 listings collected this edition (16 at 1ct, 28 at 1.5ct, 30 at 2ct) against 402 in August (77, 147, 178). A live re-query of the retailer's own inventory API on 2026-09-17 (the same day as the collection) returned near-identical counts (16, 29, 30), confirming the drop is a genuine change in listed stock rather than a collection issue. Same-day correction (2026-09-18): Quality Diamonds' cut filter has never actually restricted its raw collection to Excellent-cut, despite this index's stated Excellent-cut benchmark — the retailer's API parameter that was assumed to do this (SelectedCutVals) does not, and the raw collection mixes Excellent and Ideal cut stones (this edition: 26 Excellent, 48 Ideal, of 74). Diamond Labs' scraper already filters client-side for the identical reason (its own API mixes Excellent and Ideal too); Quality Diamonds never had that same filter. Fixed in the scraper and reapplied to this edition's already-captured data: filtered to Excellent-only, Quality Diamonds' three cells fall to n=6, n=11, n=9, below the 30-listing minimum at every weight, so none are published this edition. Checked against August's archived data for scale: the same filter would have left August's 1.5ct (n=147 to n=84) and 2ct (n=178 to n=71) cells comfortably published with prices within £1/ct of what was actually published, but would have made August's published 1ct figure (n=77 to n=17) thin. August's published figures are not being restated — every edition is a frozen snapshot once published — so this fix applies from this edition forward only. Diamond Labs and 77 Diamonds both cleared the minimum at every weight, unaffected by this fix; both collections were made September 17, 2026, with no feed issues.

Per carat, ex-VAT, August to September: Diamond Labs +2% at 1ct (£202 to £207), +2% at 1.5ct (£168 to £172), +9% at 2ct (£158 to £172); 77 Diamonds +7% at 1ct (£234 to £251), +5% at 1.5ct (£229 to £241), +2% at 2ct (£230 to £234); Quality Diamonds not comparable at any weight this edition (all three cells fell below the publication minimum once correctly filtered to Excellent-cut, see Coverage this edition). Published Market Medians: 1ct £275, down 2% from August's three-retailer £281, now on two retailers (Diamond Labs, 77 Diamonds); 1.5ct £371, down 4% from £387, also now on two retailers; 2ct £487, up 5% from August's three-retailer £466, also now on two retailers. None of the three month-over-month Market Median moves this edition are a clean like-for-like comparison: all three now draw on a different, smaller retailer set than August's.

This edition tracks three retailers: Diamond Labs, 77 Diamonds, and Quality Diamonds. The panel was established in May 2026, when eight retailers were evaluated and three qualified, and it is unchanged for August 2026. Two retailers (Brilliant Earth UK and James Allen) were screened out before formal audit: both price in GBP via USD conversion rather than independently setting UK prices. Six retailers proceeded to formal audit against four criteria: (1) live filterable inventory API with no credentials required, (2) publicly visible per-listing GBP pricing, (3) IGI certification as a filterable and per-listing attribute, and (4) minimum 30 qualifying listings per carat cell. Three qualified. Three were excluded; exclusion reasons are documented in the Excluded retailers note below. The panel is fixed and disclosed for comparability. Any additions, removals, or reclassifications will be explicitly noted in that month's report.

Queensmith (queensmith.co.uk) passed structural eligibility criteria but stocks exclusively D, E, and F color lab-grown diamonds: a live API query found zero G-color listings across all three benchmark carat bands, consistent with their premium-positioning sourcing standard. Purely Diamonds (purelydiamonds.co.uk) and Angelic Diamonds (angelicdiamonds.com) are finished jewelry retailers with no loose diamond search inventory.

Prices are the publicly visible list price at the time of data collection. Published figures in the index table include 20% UK VAT applied to the collected ex-VAT price. Ex-VAT medians (per carat) for September 2026: Diamond Labs 1ct £207 (n=64), 1.5ct £172 (n=273), 2ct £172 (n=45). 77 Diamonds 1ct £251 (n=229), 1.5ct £241 (n=775), 2ct £234 (n=265). Quality Diamonds below the publication minimum at every weight this edition once correctly filtered to Excellent-cut (n=6, n=11, n=9; see Coverage this edition). The following are excluded: promotional codes, membership discounts, negotiated pricing, and strikethrough compare-at or was-£X reference prices.

The benchmark specification is G color, VS1 clarity, round brilliant, Excellent cut, IGI-certified. G color was chosen because it is the highest-volume specification available with consistent depth across the tracked UK retailers. Three weight cells are tracked: 1ct (0.95-1.05ct), 1.5ct (1.45-1.55ct), and 2ct (1.95-2.05ct). Note: the Rings.com US price index uses E color rather than G color. E is a higher color grade and commands a price premium. Cross-market comparisons between the US and UK indices should account for this grade difference.

Each published figure is the median total stone price across all qualifying listings within that cell. Median is used instead of an average to reduce the influence of extreme outlier listings. A minimum of 30 qualifying listings per retailer is required for that retailer's cell to be published, and a Market Median requires at least two qualifying retailers in that cell; a single retailer's price is shown individually but is not published as a market figure. Per-cell sample sizes (n) are visible as tooltips when hovering over any price in the table. Quality Diamonds fell below the retailer threshold at every weight this edition once correctly filtered to Excellent-cut (n=6, n=11, n=9; see Coverage this edition); Diamond Labs and 77 Diamonds met the threshold at every weight.

Listings are deduplicated within each retailer by URL. Stones with identical IGI certificate numbers may appear at more than one retailer and are counted as independent listings at each retailer where they appear.

The UK index is built from live API queries against each retailer's product inventory backend. All three APIs are public-facing endpoints that power the retailers' own consumer-facing search pages. No credentials, scraper accounts, or private API keys were used.

The UK online lab-grown market is tracked here across three retailers (Diamond Labs, 77 Diamonds, and Quality Diamonds): the primary UK-accessible online retailers with independently-priced GBP inventory and sufficient benchmark-matching stock to produce stable medians across all three carat tiers. A pricing decision by a single UK retailer will move the index median more than an equivalent decision would move the US index.

The UK and US indices are not directly comparable: the UK index uses G VS1 and reports in GBP inc-VAT; the US index uses E VS1 and reports in USD. Historical index figures may not be directly comparable when the benchmark specification, the retailer panel, or the set of retailers with published cells changes; this edition's Market Medians all draw on two retailers (Diamond Labs, 77 Diamonds) instead of three, after Quality Diamonds' listed inventory fell below the publication minimum at every weight (both a genuine inventory contraction and a same-day fix to the site's Excellent-cut filter; see Coverage this edition).

Rings.com receives no compensation from the retailers included in this index for inclusion or ranking, and no retailer has any editorial control over the methodology or results. This index reflects public list prices only; individual purchase prices may differ due to custom configurations, time-sensitive promotions, or VAT treatment at purchase.