What does 'Type IIa' mean on a lab-grown diamond certificate?

It is a classification of nitrogen content, not a grade. Type IIa diamonds contain almost no nitrogen. Most CVD-grown and many HPHT-grown colourless diamonds are type IIa because that is how they are made, so the label is routine for lab-grown and is not a mark of rarity, whatever a listing implies.

What does 'may include post-growth treatment' mean?

It means the laboratory could not confirm the stone is untreated. Many CVD diamonds grow brownish and are heated under high pressure to remove the tint. The treatment is permanent and does not affect durability, but an 'as grown' stone has not needed it, and some buyers prefer that.

How do I check the inscription matches the certificate?

Ask the jeweller to show you the girdle under a 10x loupe or microscope; the report number is etched there, with 'LG' before it on IGI stones since 2023. Then enter the number on IGI's or GIA's online verification page and compare carat weight and measurements to the report you were handed.

How do I read a lab-grown diamond certificate?

A laboratory-grown diamond report looks like a natural diamond report and is read the same way for cut, colour, clarity and carat. The lines that matter are the ones that are specific to lab-grown, and they are easy to skim past. Take them in order.

1. The title: it must say "laboratory-grown"

The report's heading will read "Laboratory Grown Diamond Report" (IGI) or "Laboratory-Grown Diamond Report" (GIA). That wording is the document's whole point. Under the NAJ Diamond Terminology Guideline, which UK Trading Standards treat as assured advice, "diamond" on its own means a natural diamond, and a lab-grown stone must carry the qualifier on all paperwork ¹¹. A report that does not say laboratory-grown is either for a natural diamond, in which case the stone in front of you may not be the stone on the paper, or from a laboratory that is not following the rules.

2. The report number and the girdle inscription

Every IGI and GIA lab-grown report carries a number that is laser-inscribed on the stone's girdle ¹. Matching the two is the single most useful check you can make. Ask to see the girdle under a loupe; the inscription is small but readable. Then verify the number online through IGI's report-verification page or GIA's Report Check ⁹ ¹⁰ and compare carat weight and millimetre measurements.

One change to know about: until February 2023 IGI inscribed "LAB GROWN" on the girdle alongside the number. It now inscribes only its logo and "LG" followed by the report number ⁴. So an IGI inscription reading "LG 6xxxxxxx" is a lab-grown disclosure; a stone with no "LG" and no laboratory logo may still be lab-grown but has not been inscribed as such.

3. Growth method: HPHT or CVD

IGI reports state the growth method. This affects very little for the wearer, and the HPHT vs CVD page says why in a few hundred words. It matters here only because it tells you what the next line is likely to say.

4. The treatment statement

Since January 2021 IGI's lab-grown reports state one of two things: that the stone is "as grown with no indication of post-growth treatment", or that it "may include post-growth treatment" ² ³. The background is that CVD diamonds often grow with a brown tint and are then annealed under high pressure and temperature to remove it; GIA's research summary notes that since 2020 roughly 80 per cent of CVD-grown diamonds submitted to it have undergone post-growth processing ⁷. Treatment is permanent and does not weaken the stone, but it is material information, and the NAJ Guideline expects it to be disclosed. "As grown" is the cleaner statement and costs no more; if you are choosing between two otherwise identical stones, prefer it.

5. "Type IIa" is not a grade

Listings love to say a stone is "Type IIa, the rarest and purest type of diamond". On a natural diamond that is a meaningful rarity. On a lab-grown diamond it is routine: type IIa simply means almost no nitrogen, and most CVD and many HPHT colourless stones are grown that way by design ⁸. Treat it as a description, not a selling point, and do not pay extra for it.

6. The grades, and what a GIA report now leaves out

On an IGI report you then read cut, colour (D to Z), clarity (FL to I3), polish, symmetry and fluorescence exactly as on a natural report ¹. The colour and clarity page explains which grades are worth paying for in lab-grown.

On a GIA report issued since 1 October 2025 those lines are absent. GIA now confirms the stone is laboratory-grown and places it in a "premium" or "standard" band, defined by a combination of colour, clarity and finish; stones below the minimum get no band ⁵ ⁶. If you are handed a GIA lab-grown report with full grades, it predates October 2025, which is fine; it is still a valid report.

7. The date and the plot

Check the report date. Reports do not expire, but an old report on a stone that has since been recut will not match the measurements. On IGI reports a clarity plot shows where inclusions sit; for lab-grown the common ones are small metallic inclusions in HPHT stones and pinpoint clouds in CVD, neither of which matters if the clarity grade is VS or above.

If anything does not match

A mismatch between inscription and report, a report that omits "laboratory-grown", or a jeweller who will not let you loupe the girdle are each reason enough to stop. The report is the only thing separating a lab-grown diamond from a diamond simulant in a buyer's hands, and UK law, as the advertising-law page sets out, puts the duty of accurate description on the seller.

Written by Laura Chavez, Founder of Lark & Berry. Published Sun Aug 23 2026 00:00:00 GMT+0000 (Coordinated Universal Time).