Lab-grown or natural? What a photograph can and cannot show

The same material, grown two ways

A lab-grown diamond is a diamond. It is carbon in the same cubic lattice as a stone that crystallised in the mantle, with the same hardness, the same refractive index, the same dispersion and the same thermal conductivity. It is not an imitation. Cubic zirconia and moissanite are simulants: different materials that borrow the look. A lab-grown diamond borrows nothing, because it is the thing itself, grown in a chamber over weeks rather than in rock over a span nobody can picture.

That one fact governs everything else. Because the two are the same material, every test that works by measuring a property of the material fails to separate them. A jeweller’s thermal probe passes both. The fog test – breathing on the table and watching the mist lift – passes both. Scratching glass shows only that the stone is harder than glass, which is true of diamond, moissanite and sapphire alike. Weight, specific gravity, fire and brilliance all read the same.

What a photograph can and cannot do

A photograph is useful evidence about several questions and no evidence at all about this one. It can often distinguish a diamond from a simulant, because simulants differ in material and give themselves away: moissanite shows doubled facet edges seen through the crown at the right angle, cubic zirconia disperses light more broadly and its facet junctions wear round, glass shows bubbles. Those signs are set out in our guide to telling a real diamond from a simulant.

What no photograph records is growth history. Origin is written in how the crystal grew and what it kept while growing, and those features are either far below the scale of a photograph or invisible to visible light altogether. Carat reports each stone as natural, lab-grown, simulant, treated or unknown with its reasoning attached, and where a photograph supports nothing better than unknown, that is what it should say.

HPHT and CVD: two ways of growing

Laboratory diamonds are made by two routes, and the route leaves traces a laboratory can read.

High pressure, high temperature (HPHT) reproduces mantle conditions in a press, growing a crystal from carbon dissolved in a molten metal flux. Stones grown this way sometimes hold tiny metallic inclusions from that flux, which occasionally make a stone react faintly to a strong magnet, and colour can sit in blocky growth sectors rather than in the banding seen in natural stones.

Chemical vapour deposition (CVD) grows the crystal in layers from a carbon-bearing gas onto a flat seed plate, which can leave fine striations and strain aligned in one direction. As-grown CVD material is often brownish, and much of it is treated afterwards to remove the tint, routinely by an HPHT anneal. A laboratory report will note that.

None of this shows in a photograph. The inclusions are microscopic, the strain needs polarised light, and the sector structure appears only under deep ultraviolet. Knowing it explains why a laboratory can answer the question, not how you might.

The inscription on the girdle

The practical shortcut is the girdle, the narrow band around the widest part of the stone. Most laboratory-graded stones carry a laser inscription there: a report number, and for laboratory-grown material usually the words lab grown or an abbreviation, since the main grading laboratories require origin to be stated. It is microscopic but readable with a 10x loupe.

Hold the loupe close to your eye and bring the stone up to it, with a bright light off to one side rather than behind you, and rotate the stone slowly through a full turn while watching the girdle. Prongs cover part of it, so the inscription may be hidden.

Two warnings. An inscription is a record rather than proof: use it by typing the report number into the issuing laboratory’s online lookup and checking that the weight, measurements and grades match the stone in front of you. And the absence of an inscription proves nothing. Older stones, recut stones and stones never submitted for grading carry none at all.

What actually settles it

Separating lab-grown from natural is routine laboratory work, and it cannot be done anywhere else. The instruments do three things:

• Deep ultraviolet fluorescence imaging. Under short-wave ultraviolet a stone’s growth structure lights up – concentric zoning in natural stones, sector or layer structure in grown ones – and the afterglow differs too.
• Spectroscopy. Photoluminescence at very low temperature reveals defect centres whose presence and ratios point to how the crystal grew and whether it was treated. Infrared spectroscopy sorts stones by nitrogen content into types, some common in nature and rare in the laboratory.
• Magnification. Metallic flux inclusions, seed remnants and natural mineral inclusions each tell their own story at 10x and above.

Traders use screening devices that sort quickly into pass and refer. A refer is not a verdict; it means the stone needs full testing. The conclusive answer is a report from a recognised gem laboratory, and for a stone of real value that report is what a buyer, a valuer or an insurer will ask to see.

Why lab-grown resale is so much weaker

The two stones are the same material but not the same asset, and the reason is supply. Natural supply comes out of a finite number of mines on a schedule nobody controls. Lab-grown supply is a manufacturing question: add reactors, add output. Production costs have fallen steeply as capacity has grown, and retail prices have followed them down year after year.

That has a blunt consequence for reselling. A second-hand buyer prices against what an equivalent new stone costs today, not against what you paid, and today’s equivalent is cheaper than last year’s. A lab-grown diamond bought a few years ago typically recovers only a small fraction of its purchase price, and many dealers decline to buy them at all except against a trade-in.

Natural diamonds are no savings account either: retail includes the retailer’s margin, and the gap between shop price and dealer offer is wide. The difference is that natural resale is anchored to a supply nobody can expand at will. The mount is unaffected, since the metal holds its melt value whatever sits in it, which is one reason a piece is best valued in three markets.

What to ask, and what an app can add

If you are buying, four questions do most of the work. Is there a report, and from which laboratory. What is the report number, so you can look it up before you pay. Is the stone inscribed. And does the receipt state the origin in writing, since that is what you would rely on if it later turned out to be wrong.

If you already own the piece and have no paperwork, read the hallmark and any inscription, ask a jeweller to confirm the stone is diamond rather than a simulant, and send it to a laboratory if the origin question carries money with it. A reading from Carat can tell you what the piece is, what the marks say, how the stone is cut and set, and where the evidence stops – and it names the test that would settle the rest. It is a starting point for that conversation, not a substitute for it.