SearchHow to tell if a diamond is real: five checks at home, and what a tester misses
From the counter at TraxNYC, 47th Street.
The short answer
- Real means diamond, not a look-alike. A lab-grown diamond is diamond too, so every home check and tester reads it as diamond. Lab-grown or natural takes the paper, a screening machine or a gem lab.
- Five checks at home rule out the easy fakes: the paper, the stamp, the fog test, a 10x loupe and, for a loose stone, the read-through. None proves a diamond alone.
- A pen tester reads heat. Glass and cubic zirconia fail; lab-grown and natural diamonds pass the same, and so can one look-alike stone.
- Buying? Read the origin on the listing, see the stone, and keep the paper.
Not sure about a stone? Text us or call 212-872-9692. Our team texts back 10 AM to 10 PM ET; later texts are answered the next day.
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What each check can tell you
| Check | What it can tell you | What it cannot |
|---|---|---|
| The paper | What the stone was sold as, and its grades | That the stone in hand is that stone, without a matching inscription |
| The stamp | Karat gold or platinum; C.Z. means cubic zirconia | Anything about the stone itself |
| The fog test | Glass and cubic zirconia hold fog longer | Lab-grown or natural |
| A 10x loupe | Worn or doubled facet edges point to a look-alike | Lab-grown or natural |
| The read-through | Print shows through most glass and cubic zirconia | Anything about a set stone |
| Pen tester | Glass and cubic zirconia fail | Lab-grown or natural; one look-alike passes |
| Multi-tester | Catches most of that look-alike | Lab-grown or natural; a few diamonds trip it |
| Screening machine, gem lab | Lab-grown or natural | A machine refers some stones to a lab |
A diamond is crystal carbon, 10 on the Mohs hardness scale. A look-alike only looks like one: glass, cubic zirconia, white sapphire. A lab-grown diamond is diamond, and the FTC's Jewelry Guides ask for a word like laboratory-grown right before the word diamond.
Five checks you can do at home
- Read the paper and the listing. A gem lab's grading report names the stone and whether it is laboratory-grown. Reading one, and matching the number inscribed on the stone's edge, is in our lab-grown diamond guide.
- Look at the setting. Most diamonds of any size are set in karat gold or platinum: look for 10K, 14K, 18K, 417, 585, 750 or PLAT. A C.Z. stamp is a plain answer. A big stone in a plated or unmarked setting is a reason to ask questions.
- Breathe on it. Fog the clean stone with your breath. Diamond moves heat away fast, so the fog clears in a second or two; glass and cubic zirconia stay fogged longer. A look-alike that carries heat well can clear fast too.
- Use a 10x loupe. Diamond is hard, so its facet edges stay crisp; cubic zirconia is softer, and its edges round and scratch with wear. Edges that look doubled point to a look-alike. Tiny marks inside are a hint, not proof.
- The read-through test, loose stones only. Lay a loose round stone face down on print. Through a well-cut diamond you cannot read the letters, since the facets send the light back up; through glass or cubic zirconia you often can.
Skip these. Scratching glass proves nothing, since cubic zirconia scratches it too. Heating a stone and dropping it in cold water can crack one with marks inside.
What a diamond tester reads, and what it misses
A pen tester reads how fast heat moves from its tip into the stone. Diamond carries heat fast, so natural and lab-grown diamonds both read as diamond; glass and cubic zirconia fail.
One look-alike stone carries heat well enough to pass a heat-only pen. A multi-tester adds electricity, which flags most of it; a few diamonds conduct too, natural blue ones and some lab-grown ones, and can trip the alert. Neither tester can tell lab-grown from natural; that takes a screening machine or a gem lab.
The pen tester. A lab-grown diamond chain and a natural diamond chain: both read as diamond. 1.3 million views on YouTube, read September 26, 2026.
Buying a diamond: what the listing and the paper should say
Nearly all of our diamond listings say lab-grown diamond or natural diamond; where one does not, ask the desk before you pay. Our showroom page puts the rest plainly: "Every stone is shown in person or on video before you decide."
It adds: "Every purchase comes with an official appraisal: carat and gram weights, diamond grade and color, current market value, and our corporate seal." Silver jewelry is the exception. An appraisal is not a lab report; if you want one for a stone, ask before you order.
The metal has its own promise. If anything you bought from us ever tests under the metal it was sold as, a gold karat or sterling silver, you get your money back in full, with no time limit. Text or call 212-872-9692 with your order number.
Below, two pairs: natural and lab-grown. Every home check and tester reads all four as diamond; the listing says which is which.
Shop both kinds: tennis chains, men's diamond chains, diamond stud earrings, engagement rings and lab-grown diamond jewelry.
From the counter
“The jewelry business is about trust. You will not learn everything there is to know before you buy, so buy from someone who will still be around, and who treats you with dignity. I have seen customers start over with a new jeweler because they picked the one who fooled them best: the one who made them feel good at the counter and bad once they understood what they bought.”
Text us or call
Buying a diamond? See it before you decide.
Every stone is shown in person or on video.
Questions we hear at the counter
Can a diamond tester be wrong?
It answers a narrower question than people think: a heat-only pen reads lab-grown and natural diamonds the same, and one look-alike passes it. A gem lab settles it.
Does a real diamond sparkle like a rainbow?
A diamond throws white light and flashes of color together. A stone that throws far more rainbow than white may be a look-alike: cubic zirconia splits light into color more than diamond does.
Do real diamonds get cloudy?
A diamond does not cloud by itself; a film of skin oil and lotion on its back dulls it, and a wash brings it back: how to clean gold jewelry.
How can I tell if my diamond is lab-grown or natural?
Not at home. Start with the paper and any inscription on the stone's edge; without them, a screening machine or a gem lab decides. Our lab-grown diamond guide walks through both.
Keep reading
More answers from the counter: all TraxNYC buyer's guides.
See the stone at the counter
64 W 47th St, Booth F-1, New York, NY 10036, on 47th Street since 2004. Monday to Friday 11 am to 6 pm ET, Saturday and Sunday 12 pm to 4 pm ET; around a holiday, text first.
Call or text 212-872-9692, or read our showroom page and our reviews page first.