Lab-grown diamond vs cubic zirconia: are they the same?
No, a lab-grown diamond and cubic zirconia are not the same. A lab-grown diamond is real diamond: carbon, 10 on the scale, and it reads as diamond on a thermal diamond tester. Cubic zirconia (CZ) is zirconium dioxide, a cheap man-made diamond simulant at 8 to 8.5 that scratches and clouds with wear and fails a thermal diamond tester.
Is a lab-grown diamond the same thing as cubic zirconia?
No. A lab-grown diamond is diamond. The US Federal Trade Commission’s Jewelry Guides define diamond as a mineral of essentially pure carbon crystallised in the isometric (cubic) system, with a hardness of 10, a specific gravity of about 3.52 and a of 2.42 (16 CFR 23.12), and GIA says lab-grown diamonds have the same chemical, physical and optical properties as natural diamonds. Cubic zirconia is a different material: zirconium dioxide (ZrO₂), which GIA classes as a diamond simulant, a stone that copies diamond’s look but not its properties. In GIA’s words, diamond simulants are not the same as laboratory-grown diamonds. The mix-up starts because both are made in labs: natural cubic zirconia is very rare, and the CZ in jewellery is lab-made, from powder melted and then cooled into a solid crystal. Being made in a lab does not make a stone a lab diamond; being diamond does. For how real diamond is grown, see how lab gems are grown.
How do lab diamond and cubic zirconia compare?
| Lab-grown diamond | Cubic zirconia (CZ) | |
|---|---|---|
| What it is | Diamond: carbon crystallised in the cubic system, the same as a mined diamond | Zirconium dioxide, a man-made diamond simulant |
| Hardness (Mohs) | 10 | 8 to 8.5 |
| Refractive index (brightness) | 2.42 | 2.15 to 2.18 |
| Dispersion (rainbow fire) | 0.044 | 0.058 to 0.066: slightly more fire, less brilliance |
| Specific gravity (weight) | 3.52 | 5.56 to 6.00: much heavier for its size |
| Thermal diamond tester | Reads as diamond | Reads as not diamond; CZ is a thermal insulator |
| Price | Many times the price of a CZ the same size | A small fraction of a lab diamond’s price |
| With years of wear | Harder than CZ and every other simulant, so it resists scratches far better | About two years of regular wear before it looks cloudy and scratched; colourless CZ may yellow |
| What a US listing should call it | Laboratory-grown, laboratory-created or [maker]-created diamond | Cubic zirconia, or simulated or imitation diamond |
Hardness, refractive index, dispersion and specific gravity are from the comparison table in GIA’s journal Gems & Gemology (Winter 1997), which gives diamond’s refractive index as 2.417 (the FTC rounds it to 2.42); CZ’s figures vary a little because its composition varies. The wear figure for CZ and the price gap are from the International Gem Society, and the yellowing note is GIA’s.
Is cubic zirconia a lab-grown diamond?
No. CZ is lab-made, but it is not diamond, so it is not a lab-grown diamond. Section 23.25(c) of the FTC Jewelry Guides says words such as "laboratory-grown", "laboratory-created" or "synthetic" should be used with a natural stone’s name only if the product has essentially the same optical, physical and chemical properties as the stone named. CZ fails that test: its hardness, refractive index, dispersion and specific gravity all differ from diamond’s (see the table above). For imitation diamonds, section 23.12 points to section 23.25, and 23.25(b) says a stone’s name used for an imitation or simulated stone should come straight after a word such as "imitation" or "simulated", as prominent as the name itself, so "simulated diamond" is an honest description of CZ and "lab diamond" is not. Section 23.12 adds that using the word diamond on its own for anything that does not meet the definition of diamond is unfair or deceptive. A lab-grown diamond may be called a diamond, as long as the lab-grown part is clear: 23.12 allows "cultured" only with a clear qualifier such as "laboratory-grown" or "laboratory-created". So if a listing says "lab created diamond" but its details say cubic zirconia or CZ, the stone is CZ. For the rest of the vocabulary, see lab-created vs simulated.
Which lasts longer, and which sparkles more?
The lab diamond lasts longer. At 10 on the Mohs scale, diamond is harder than CZ (8 to 8.5) and every other simulant in GIA’s comparison table. GIA says CZ scratches more easily than diamond and that colourless CZ may yellow over time. The International Gem Society says household dust can mark CZ, that it absorbs oils from skin and skin products, and that it typically keeps its looks for only about two years of regular wear before it turns cloudy and scratched. On sparkle, GIA says CZ has slightly more fire but less brilliance than diamond: its dispersion is higher (0.058 to 0.066 against 0.044) and its refractive index lower (2.15 to 2.18 against 2.42). GIA adds that some people find CZ’s broad flashes of colour look artificial. Our cubic zirconia page compares CZ with diamond and moissanite and has photos of all three stones.
How can you tell a lab diamond from cubic zirconia?
A thermal diamond tester. Diamond conducts heat very well and CZ is a thermal insulator, so a basic thermal tester reads a lab diamond as diamond and a CZ as not diamond. GIA’s journal notes that jewellers have long relied on these probes to separate diamond from its simulants; the exception is moissanite, which also reads as diamond (see does moissanite pass a diamond tester). Weight. CZ is much denser (specific gravity 5.56 to 6.00 against 3.52), so a CZ cut to the same size and shape as a 1 ct diamond weighs about 1.6 to 1.7 ct. Put the other way, as GIA does, a simulant denser than diamond can look smaller than a diamond of the same carat weight, so a "1 carat CZ" is a smaller stone than a 1 carat diamond. Wear and magnification. An old CZ often looks cloudy and scratched, and under magnification CZ can show tiny gas bubbles and unmelted zirconium oxide powder. The report. A lab diamond sold with an IGI or GIA report number can be looked up on the laboratory’s website: see how to check a diamond report number. No home test shows whether a diamond was grown or mined: GIA says a lab-grown diamond cannot be spotted with a loupe and only laboratory testing can tell. That is what the report is for.
Which should you buy?
Buy CZ for fashion jewellery, a costume piece or to try a look cheaply. It costs a small fraction of a lab diamond the same size, and it is honest as long as it is sold as CZ. Buy a lab diamond for an engagement ring or anything worn every day for years, ideally with an IGI or GIA report you can check. If CZ wears too fast and a lab diamond costs too much, moissanite (Mohs 9.25) sits between them on hardness: see moissanite vs cubic zirconia and moissanite vs lab diamond. For what a lab diamond costs, see how much a 1 carat lab-grown diamond costs.
Frequently Asked Questions
Is cubic zirconia a lab-grown diamond?
No. CZ is made in a lab, but it is zirconium dioxide, not diamond. Under the FTC Jewelry Guides (16 CFR 23.25), "lab-grown" or "lab-created" belongs with a stone’s name only if the product has essentially the same optical, physical and chemical properties as that stone, so CZ should be sold as cubic zirconia, or as a simulated or imitation diamond.
Is cubic zirconia a fake diamond?
It is an imitation of diamond, which GIA calls a diamond simulant. That is not dishonest in itself: CZ sold as CZ is fairly described. It only becomes a fake when someone sells it as diamond.
Will cubic zirconia pass a diamond tester?
No. CZ is a thermal insulator, so a thermal diamond tester reads it as not diamond. A lab-grown diamond passes, exactly as a mined diamond does, because it is diamond.
Does cubic zirconia get cloudy?
Yes, with wear. The International Gem Society says CZ typically keeps its looks for about two years of regular wear before it turns cloudy and scratched, and GIA notes that colourless CZ may yellow over time. Diamond, at 10 on the Mohs scale against 8 to 8.5 for CZ, does not scratch the same way.
Sources
- US FTC Jewelry Guides, 16 CFR 23.12: definition and misuse of the word diamond (Cornell LII)
- US FTC Jewelry Guides, 16 CFR 23.25: laboratory-grown, synthetic, imitation and simulated stones (Cornell LII)
- GIA 4Cs: What are simulated diamonds? (CZ vs lab-grown diamond)
- GIA 4Cs: How to tell if a diamond is real (lab-grown diamonds are real diamonds; only laboratory testing tells them from mined)
- GIA, Gems & Gemology (Winter 1997): Synthetic moissanite, a new diamond substitute (Table 1: diamond and CZ properties and inclusions; thermal probes)
- International Gem Society: Cubic zirconia vs diamonds (zirconium dioxide, wear, dust, about two years, price gap)
- Wikipedia: Cubic zirconia (natural rarity, thermal insulator, how it is made)
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