Let's say it straight away, because it is the only thing that matters. A thermal conductivity diamond tester does not tell a lab-grown diamond from a mined one. Plenty of pages claim otherwise. That is wrong. These devices answer "diamond" or "not diamond", nothing more. This article explains what actually works, how to read an IGI report, and why coloured diamonds deserve extra attention.

With the naked eye and with a loupe: impossible

A lab-grown diamond is crystallised carbon. So is a mined diamond. Same structure. Same hardness of 10. Same refractive index. Same dispersion, so the same fire.

There is no visual difference between the two. Not in colour, not in brilliance, not in the way they handle light.

One point deserves to be said plainly, because a lot of content implies the opposite. A jeweller does not "see" the difference. Even under a 10x loupe, looking at inclusions gives no reliable answer. Some inclusions are statistically more common on one side than the other. Common does not mean conclusive.

This impossibility is not a problem. It is the proof that a lab-grown diamond really is a diamond. A simulant, on the other hand, can be spotted. We cover that in our article on how to recognise a real diamond.

The myth of the thermal conductivity diamond tester

Here is the single worst covered point on the whole web, and the main angle of this article.

A pocket tester measures the thermal conductivity of the stone. Diamond conducts heat better than almost any other material. The device applies a heated tip and measures how fast the heat is drawn away.

What the tester tells you: this stone is diamond, or it is not. It rules out cubic zirconia and glass, which conduct poorly.

What the tester does not tell you: whether the diamond was grown or mined. And for an obvious reason. Both are diamond. They conduct heat in exactly the same way. The machine has no physical means of separating them.

Dual probe models add an electrical conductivity reading. That one is there to rule out moissanite, which passes the thermal test. It says nothing about where the stone came from.

No consumer tester displays "lab-grown" or "natural". If a seller claims to have used a tester to confirm that a stone is natural, they are mistaken or they are misleading you.

Professional screening instruments do exist. Their logic is different: they separate stones consistent with a mined diamond from those that need to go to a laboratory. A "refer" result is a direction to follow, not a verdict.

The girdle inscription: the real clue

The girdle is the edge of the stone, the band that separates the crown from the pavilion. That is where laboratories laser inscribe their marking.

This inscription is microscopic and invisible to the naked eye. You need a good quality loupe, often a microscope, and grazing light. Another limit: on a set jewel, the prongs can cover the inscription and make it unreadable without unmounting the stone.

The content of the inscription has changed, and most pages have not caught up.

At IGI, the full wording has given way to an abbreviated form. The inscription now consists of the IGI logo, the letters "LG" and the report number. A recent IGI stone may therefore carry no readable word at all. That does not mean it is not certified. You need to note the number and check it online. Writing "just look for the words LAB GROWN on the girdle" is no longer accurate for IGI stones.

GIA, the other leading gemmological laboratory, uses its own inscription together with its own report number. Conventions differ from one laboratory to another. So the first thing to establish is which laboratory issued the report.

At Lauredi, our stones are IGI certified. It is the IGI verifier you should query for our jewellery.

A five minute check, with the jewel in your hand

The practical method needs nothing more than a loupe and a phone.

Step 1. Identify the issuing laboratory, shown on the report handed over at purchase. IGI, GIA, HRD: each one has its own online verifier.

Step 2. Look for the girdle inscription. High magnification loupe, grazing light, slow rotation. Write the number down. If the setting hides the girdle, go to step 3.

Step 3. Enter the report number in the online verifier of the issuing laboratory. The full report appears.

Step 4, the most important one and the most often skipped. Compare the report on screen with the stone in your hand. Does the weight match? Is the cut the same shape? Do the millimetre measurements line up? An authentic report paired with a different stone is a known case. The number on its own proves nothing.

Step 5. If the inscription is unreadable or absent and the doubt remains, there is only one way out: have the stone analysed by a gemmological laboratory. A jeweller can arrange that submission.

Reading a lab-grown diamond report in 2026

The certification landscape has shifted recently. Two changes appear in almost no English language content.

First change, at GIA. In autumn 2025, GIA changed its reporting system for lab-grown diamonds. The classic 4C grades give way to two descriptive categories, "Premium" and "Standard". A recent GIA report on a grown stone therefore no longer reads like a mined diamond report. Check the issue date of the document before you interpret what it says.

Second change, at IGI. IGI has modified the marking inscribed on the girdle. The full wording has given way to an abbreviated inscription, along the lines of "LG" followed by the report number, together with the laboratory logo. An IGI stone can therefore be perfectly certified without carrying any explicit wording. IGI also continues to issue reports with colour and clarity grades.

These policies keep evolving. We describe them as we understand them today. If you have any doubt about a specific document, it is the online verifier of the issuing laboratory that settles the matter, not a blog article.

What only a gemmological laboratory can settle

When there is neither a report nor a readable inscription, the only reliable answer comes from a properly equipped laboratory. Here is what happens there, without needless jargon.

Photoluminescence. The stone is cooled and excited with a laser. Diamond re-emits light through signatures linked to its atomic defects. Those signatures differ depending on how the stone formed.

Infrared and ultraviolet visible spectroscopy. The way the stone absorbs light is measured. The absorption profile reveals the amount and configuration of nitrogen and boron.

Deep ultraviolet fluorescence imaging. The most telling method. The growth patterns of the crystal become visible. A diamond produced by chemical vapour deposition shows parallel striations. A diamond produced under high pressure and high temperature shows geometric sectors.

Secondary clues exist, though none of them ever proves anything on its own. Some stones produced under high pressure contain small metallic inclusions from the growth flux, sometimes responsive to a strong magnet. Unusual phosphorescence can also draw attention. These are signals, not verdicts.

Coloured diamonds, the case nobody covers

Everyone online talks about white diamonds. Yet it is with coloured stones that the question really matters.

First issue, money. The price gap between a mined coloured diamond and a lab-grown coloured diamond is far wider than it is on white. An intense natural yellow or blue is extremely rare in the ground. Getting the origin wrong therefore costs a great deal more.

Second issue, technical. On a coloured diamond, there are two questions instead of one. Was the stone grown or mined? And does its colour come from the growth or from a treatment applied afterwards? Post growth treatment is legitimate, but it must be declared on the report. So look for the line devoted to colour origin.

Third issue, gemmological. Analysis is more complex on a coloured stone. The elements that create the hue, nitrogen for yellow, boron for blue, leave spectral signatures that have to be interpreted. That is expert work.

At Lauredi, all our coloured stones are IGI certified lab-grown diamonds. We write it on the product page and on the invoice. Browse our diamonds or the Mme La Marquise collection.

Why this question should not worry you

A lot of content treats this subject in an anxious tone. We prefer the opposite.

If you are asking the question, it is often because you have inherited an older piece. In that case, the answer is a matter of dates. Large scale production of jewellery quality lab-grown diamonds is recent. An antique jewel very probably holds a mined stone.

If your doubt is about a seller, the real protection is not a test, it is a document. An independent laboratory report with a number you can check online, and an invoice that states the exact name of the stone. We go through those checks in our guide to buying jewellery online.

Finally, a lab-grown diamond is not a discount diamond. Same material, same hardness, same lifespan. Our article on the environmental impact of the two supply chains covers the subject in detail.

The takeaways

What does not work: the naked eye, the loupe, home tests, and above all the thermal conductivity tester. That last one rules out cubic zirconia and glass. It will never tell you whether a diamond was grown or mined.

What does work: the report number inscribed on the girdle, checked online with the issuing laboratory, then compared with the stone in your hand. Failing that, an analysis at a gemmological laboratory.

On a coloured stone, add one question. Where does the hue come from? Growth or treatment? The report has to say so.

To go further. On true simulants, which should not be confused with a lab-grown stone, read real diamonds versus simulants. On the budget question, see our breakdown of the price of a coloured diamond. And for what a brand promise really guarantees, read ethical jewellery, what the word guarantees.

FAQ

Can a diamond tester detect a lab-grown diamond?

No. A thermal conductivity tester only shows whether the stone is diamond or not. A grown diamond and a mined diamond conduct heat in an identical way, so the device is bound to confuse them. It is there to rule out cubic zirconia and glass, nothing more.

Can a jeweller spot a lab-grown diamond with a loupe?

No, not reliably. Some inclusions are more common in one type of stone than the other, but none of them is conclusive. A serious professional will point you to a gemmological laboratory rather than rule on it themselves.

How can you tell if a diamond is lab-grown without a certificate?

Start by looking for a laser inscription on the girdle, with a loupe and under grazing light. If it is missing or hidden by the setting, the only reliable route is a submission to a gemmological laboratory. A jeweller can handle that for you.

How do you read the certificate number inscribed on the girdle?

You need a high magnification loupe and grazing light, turning the stone slowly. The inscription is microscopic and invisible to the naked eye. Once you have the number, enter it in the online verifier of the issuing laboratory.

What should you check on a lab-grown diamond report?

The name of the laboratory, the report number, the explicit statement of how the stone was formed, the weight, the shape and the dimensions. Then compare that data with the stone in front of you. On a coloured stone, also check the colour origin.

Can a CVD diamond be told apart from an HPHT diamond?

Yes, but only in a laboratory. Deep ultraviolet fluorescence imaging reveals the growth patterns: parallel striations for the chemical vapour deposition process, geometric sectors for the high pressure high temperature process. No home test gives you that information.

Fiona