Does Lab-Grown Diamond Melee Pass a Diamond Tester? Pen Testers, Screeners and What Each One Says

Thermal pen testers say "diamond" — because it is diamond

A pen tester touches a heated probe to the stone and measures how fast the heat is drawn away. Diamond conducts heat better than almost any other material, so the probe cools quickly and the tester lights up "diamond". Laboratory-grown diamond has the same crystal structure and the same thermal conductivity as mined diamond, so it gives exactly the same reading. There is no setting, trick or second test on a thermal pen that separates them; the instrument was never designed to.

So when a client, an appraiser or a buyer at the next bench puts a pen to a ring set with our melee, every stone reads diamond. That is the expected result and the honest one: the stones are diamond. What the pen cannot tell anyone is where the diamond grew — and that is the part you must disclose in writing, on the invoice and the appraisal. How to word it →

Combined testers and the HPHT trap

Combined testers add a second measurement: electrical conductivity. Most diamond is an electrical insulator; the second test exists to catch a particular simulant that conducts heat well but, unlike diamond, also conducts electricity. The trap is that a small share of HPHT-grown colourless diamonds carry traces of boron from the growth process, and boron makes diamond conduct electricity too. On a combined tester such a stone can light the "not diamond" indicator even though the thermal reading said diamond.

If that happens with a stone you bought as laboratory-grown diamond, do not conclude it is a fake. Read the thermal result on its own, and if you need certainty, run the stone through a screener or send it to a laboratory: a type IIb (boron-bearing) reading is itself a sign of laboratory growth, since boron-bearing mined diamonds are among the rarest of all. CVD-grown stones do not carry boron and do not trip the electrical test.

Screening instruments: what "refer" means

Screeners such as the GIA iD100, De Beers SYNTHdetect and similar bench units do not test whether a stone is diamond; they test whether a diamond could be laboratory-grown. Almost all mined diamonds contain nitrogen in a form (type Ia) that laboratory-grown diamonds do not, so the instrument looks for that nitrogen signature. A stone that shows it passes as natural. A stone that does not — every laboratory-grown stone, and the roughly one to two per cent of mined diamonds that are nitrogen-free type IIa — is referred for laboratory testing. The screener is deliberately cautious: it never calls a laboratory-grown stone natural, and it accepts referring a few mined stones as the price of that.

So our melee will be referred, and should be. A referral is the instrument doing its job; the laboratory then confirms laboratory-grown from the growth structure under fluorescence imaging and from spectroscopy. If you sell a piece with laboratory-grown melee and the client's appraiser screens it five years from now, the stones will refer — which is fine, and expected, if the invoice and appraisal already said laboratory-grown diamond melee. It is only a problem if they said diamond alone.

Screening melee specifically

Melee is where mixing — laboratory-grown stones hidden in parcels sold as mined — has caused real trouble in the trade, and it is why melee screening exists as a service. Bench screeners test one stone at a time down to roughly 0.9 mm; automated melee screeners such as the De Beers AMS2 run thousands of loose stones an hour; and the major laboratories offer parcel screening (the GIA Melee Analysis Service and its equivalents at IGI and others) that sorts a parcel into natural, laboratory-grown and simulant, with a report for the parcel rather than a grade per stone.

For a parcel bought as laboratory-grown, screening answers a different question: not "is any of this laboratory-grown?" but "is all of it diamond?". A mixed parcel with a simulant in it is the failure mode to worry about with an unknown supplier, and a thermal pen on a sample of stones, or a screener set to refer, finds it quickly. We sell our melee as laboratory-grown diamond, DE colour, VS–VVS clarity, graded as a parcel, with a photo or video of the actual stones before dispatch and a 14-day replace-or-refund guarantee; if you want an independent check before you set a large parcel, a laboratory melee screen is the right tool, and we would rather you ran it than wondered.

What to do at the bench

• Check incoming melee the way you check any parcel. A thermal pen on a few stones confirms they are diamond. Weigh the bag, gauge a few stones against the stated size, and compare the parcel with the photo or video we sent before dispatch.
• Expect a screener to refer the stones. Write "laboratory-grown diamond melee" on the job envelope, the invoice and the appraisal so the referral never surprises anyone later.
• Do not trust a combined tester's electrical result on its own with HPHT-grown colourless stones. Thermal says diamond; a screener or a laboratory settles the rest.
• Keep laboratory-grown and mined melee in separate, labelled parcels. Once mixed they can only be separated by screening, stone by stone, at your cost.
• Setting next to a mined centre stone is the normal use and works well; the side stones match in colour, brightness and tester reading. Disclose both on the invoice. What is the same and what is different →

Questions jewelers ask

Does lab-grown diamond melee pass a diamond tester?

Yes. A thermal pen tester reads laboratory-grown diamond as diamond, because it has the same thermal conductivity as mined diamond — it is diamond. No thermal tester can tell where a diamond grew.

Why did a combined tester say my laboratory-grown stone was not diamond?

Combined testers add an electrical conductivity test. Some HPHT-grown colourless stones contain traces of boron and conduct electricity, so the electrical stage can misread them as a simulant even though the thermal stage read diamond. A screener or a laboratory test confirms the stone; a boron (type IIb) signature is itself a sign of laboratory growth.

Will a screener like the GIA iD100 flag laboratory-grown melee?

Yes — it will "refer" it, which is the correct result. Screeners pass mined diamonds that contain the usual nitrogen signature and refer everything else, including every laboratory-grown stone and the small share of nitrogen-free mined stones. A laboratory then confirms laboratory-grown from the growth structure.

Can a jeweler tell laboratory-grown from mined melee by eye or loupe?

No. The look, hardness, brightness and fire are those of diamond. Under magnification, metallic flux specks suggest HPHT growth and pinpoints or faint growth banding suggest CVD, but these are clues for a gemmologist, not a test. Only screening and laboratory analysis are conclusive.

How do I check a parcel of laboratory-grown melee is all diamond?

A thermal pen on a sample of stones catches a simulant; a laboratory melee screening service (for example the GIA Melee Analysis Service) sorts a whole parcel into natural, laboratory-grown and simulant. Weigh the bag and gauge a few stones against the stated size at the same time.

Do I have to disclose that melee is laboratory-grown if it passes a diamond tester?

Yes. Passing a thermal test only confirms the stone is diamond. Under the FTC Jewelry Guides and ISO 18323, laboratory-grown diamond must be described as laboratory-grown (or laboratory-created, lab-grown) with the qualifier next to the word diamond, on the invoice, the appraisal and the listing.