RAPI's 1-carat rises to $3,916.19 across five sessions
The index published a new point on each of 17, 18, 19 and 20 September, a rise of $4.28 from the 16th. IDEX's broad one-carat driver averaged $3,051.17 on the same morning, 22.1% below the narrow benchmark.
By The Diamond Desk
What changed
The one-carat kept printing. RapNet's one-carat index rose from $3,911.91 on 16 September to $3,916.19 on 20 September, publishing a new point on each session in between.
What it means
- If you buy one-carat polished: The benchmark rose a tenth of a per cent. The broad counter sits 22.1% under it.
- If you quote off RAPI: Carry the date you read the series, not only the date of the point.
- What this is not: A fast market. Four sessions moved the one-carat by $4.28.
Key figures
| RAPI, 1 caratpoint dated 20 September | $3,916.19 |
|---|---|
| IDEX 1ct driver | $3,051.17 |
| 16 to 20 September | +$4.28 |
| Sessions published | 5 |
Source: RAPI 1.00ct, D–H / IF–VS2, series read 22 September 2026 · IDEX price-driver average read 05:00 ET 21 September 2026
The index published daily
Five sessions, five points. RapNet's one-carat index published a reading on 16, 17, 18, 19 and 20 September. The level moved on every one of them.
Daily is the practice. The index moved from monthly to daily publication on 7 September. The five sessions to 20 September added $4.28, or 0.109%.
| Weight | RAPI | IDEX | Gap |
|---|---|---|---|
| 0.50 carat | $1,318.29 | $1,379.00 | +4.6% |
| 1.00 carat | $3,916.19 | $3,051.17 | −22.1% |
| 3.00 carat | $18,210.15 | $16,224.00 | −10.9% |
Notes on this table
Carat Capital's arithmetic. the gap column is a division, not a published figure. At one carat, 3,051.17 ÷ 3,916.19 = 77.9% of the RAPI level, which is 22.1% below it. At half a carat, 1,379.00 ÷ 1,318.29 = 104.6%, or 4.6% above. At three carats, 16,224.00 ÷ 18,210.15 = 89.1%, or 10.9% below.
They measure different things. RAPI averages the ten per cent lowest-priced D–H, IF–VS2, GIA-graded stones on RapNet, a floor for a narrow top specification. IDEX price drivers average a far broader spread of colour and clarity, so a gap at one carat is expected and is not a disagreement.
Hourly. IDEX states on its own page that the index updates every hour; a second read at 06:15 ET on 21 September showed the same driver at $3,055.60. The figure printed here is the 05:00 ET reading.
The broad counter sits lower
Two numbers, one carat. IDEX's price-driver average for a one-carat round read $3,051.17, against RAPI's $3,916.19, a gap of 22.1%.
The gap inverts. At half a carat the broad counter sits above the narrow one, $1,379.00 against $1,318.29, a gap of 4.6% the other way.
Lab-grown moved the same week
The other counter repriced. A lab-grown retail index published on 18 September reset its one-carat median to $571, a far larger move than the natural benchmark's.
What to watch
- 23 September 2026The next daily point, and whether the published series is restated again.
- Week of 26 October 2026The next lab-grown retail index edition, the counter-side comparison.
- Each 05:00 ETThe tape's read of both benchmarks, dated to each source's own newest point.
The story so far
- RAPI 1-carat adds 0.59% since 8 August, now published dailyThe switch from monthly to daily publication.
- $711, $650, $511: one lab-grown carat, three pricesWhy one carat carries several prices at once.
Go deeper
Corrections
Correction, 22 September 2026. This article as published on 21 September said RapNet's one-carat index had carried $3,912.13 since 16 September, its newest published point, with no new print in the five mornings since. The headline, the dek, the key-figure panel, a full section of the body, the homepage wire item and the entry in The Record all carried that claim. It is false. The index published a new point on each of 17, 18, 19 and 20 September. Read from the source's own embedded daily series on 22 September, those points are $3,910.74, $3,916.19, $3,915.02 and $3,916.19, and a further point dated 21 September at $3,918.53 has since been added.
Where the wrong figure came from. Not from the source. This paper's tape read $3,912.13 correctly on 17 September, dated to the source's 16 September point, and then re-stamped that same value as unchanged on four consecutive mornings without fetching the page again. The article counted those five tape entries and reported the count as a fact about the market. The source was reachable on every one of those mornings and was publishing.
What was correct as published. The 16 September reading itself, which this paper printed as $3,912.13 and the source now carries as $3,911.91; the IDEX one-carat price-driver average of $3,051.17; the half-carat driver of $1,379; and the disclosure that the IDEX index updates hourly. This paper's standards desk re-verified each of those against its own source and found them accurate.
What has changed on this page. The headline, dek, key-figure panel, body and Table I now carry the real series. The comparison figures are recomputed against the 20 September RAPI points, moving the one-carat gap from 22.0% to 22.1%, the half-carat gap from 4.8% to 4.6% and the three-carat gap from 10.0% to 10.9%. The same correction has been applied to the homepage wire item and to The Record. A separate article on the diamonds desk, dated 22 September, reports the source's restatement of its own published points.
What would change this call
A restatement. The points printed here were read on 22 September; the source has already moved them once, and a further revision would change the table without changing the direction.
Why two indices disagree by a fifth
RAPI is the average asking price of the ten per cent lowest-priced stones on RapNet within a narrow band, D to H colour and IF to VS2 clarity, GIA graded. It is a floor for a top specification rather than a market mean. IDEX price drivers average a far broader spread of colour and clarity, D to K and IF to I1 at one carat, and weight each driver by its share of the market. A broad average of an inclusive band will sit below a floor for a narrow one, which is what the one-carat row shows. At half a carat the relationship inverts because the two bands overlap differently at that weight. Neither is wrong; a single diamond price does not exist.
Method
The RAPI points here were read on 22 September 2026 from the index page's own embedded daily series, parsed out of the raw bytes of a 200 response whose last-modified header read 22 September 2026 09:15:54 GMT, not from a rendered table and not from a summary. The points used are those dated 20 September, the newest available at this article's original press time on 21 September: $1,318.29 at half a carat, $3,916.19 at one carat and $18,210.15 at three. The IDEX figures are unchanged from publication and were re-verified by this paper's standards desk against idexonline.com on 21 September; that desk confirmed the one-carat driver at $3,051.17 and the half-carat driver at $1,379.06, printed here as $1,379. The three-carat IDEX figure of $16,224 is carried from publication and was not re-read on 22 September, and is labelled here rather than folded in as though it had been. Every gap in Table I is this desk's own division and is printed alongside the sentence that uses it.