Until 2025 this was the big question in lab-grown buying guides, and the answer was the same as for mined diamonds: GIA was the stricter laboratory and IGI the commercial one. That framing is now out of date, because the two laboratories have taken different views of whether lab-grown diamonds should be graded at all.
What changed in 2025
On 2 June 2025 GIA announced that it would stop using the colour and clarity nomenclature it developed for natural diamonds when describing laboratory-grown ones 1. The new service started on 1 October 2025 2. A GIA lab-grown report now confirms the stone is a laboratory-grown diamond and places it in one of two bands, "premium" or "standard", defined by a combination of colour, clarity and finish; stones below the minimum get no designation 4. GIA's reasoning, quoted by its chief laboratory officer Tom Moses, is that more than 95 per cent of lab-grown diamonds entering the market fall into a very narrow range of colour and clarity 4. Trade commentary at the time read it as GIA stepping back from a product whose price no longer supported a full grading fee 3.
IGI went the other way. It still issues the full D-to-Z colour scale and FL-to-I3 clarity scale for lab-grown diamonds, plus cut grade, fluorescence and measurements 6, and since January 2021 its reports state whether a stone is "as grown with no indication of post-growth treatment" or "may include post-growth treatment" 7. One retailer analysis puts IGI's share of lab-grown grading at about 65 per cent against roughly 5 per cent for GIA 5; the precise figures are not published by either laboratory, but nobody in the trade disputes that IGI dominates the category.
What this means in the UK
UK retailers rarely grade diamonds here. Most lab-grown stones sold in Britain are cut and graded in India and arrive with their report, so the question is not which laboratory is nearer but which report tells you what you need to know.
An IGI report gives you a conventional colour and clarity grade to compare across retailers, the growth method (HPHT or CVD), a treatment statement, and a report number that is laser-inscribed on the girdle. Note that since February 2023 IGI inscribes only its logo and "LG" followed by the number, not the words "LAB GROWN" 8, so the inscription tells you it is lab-grown only if you know what "LG" means.
A GIA report issued after October 2025 confirms the stone is a laboratory-grown diamond and whether it is "premium" or "standard". That is a genuine assurance of identity from a laboratory with no commercial interest in the outcome. It is not a tool for comparing two stones. If you are choosing between a "premium" GIA stone and an F VS1 IGI stone, the IGI report lets you ask more questions.
Older GIA reports (pre-October 2025) carry full grades and remain valid. A UK retailer clearing older stock may still offer them.
Whichever report you are shown, verify it. IGI and GIA both run free online checks 9 10: enter the report number and compare weight, measurements and inscription to the stone in front of you. A retailer who will not let you do this under a loupe should be treated with caution.
Where the law comes in
The report is also the document that lets a UK retailer meet its disclosure duties. The National Association of Jewellers' Diamond Terminology Guideline, which UK Trading Standards have recognised as Primary Authority assured advice since December 2020, expects lab-grown diamonds to be described as such at every point of sale 11. The advertising-law page covers what the retailer must say; the certificate page covers how to read the report line by line.
The short version
For a UK buyer in 2026 an IGI report with a verified inscription is the working standard for lab-grown. GIA is not a step up for this category any more; it is a different, more minimal document. Neither laboratory's name should move the price much. What should move the price is the stone, and on that the colour and clarity page is where to go next.