Is cubic zirconia a real diamond?

No. Cubic zirconia is a diamond simulant made from zirconium dioxide. It is not a diamond and does not have diamond's carbon structure, hardness, or optical properties.

Does cubic zirconia get cloudy over time?

Yes. Cubic zirconia is more porous than diamond and accumulates oils, residue, and surface scratches with daily wear. Most CZ stones develop visible cloudiness within two to five years of regular use.

Can cubic zirconia pass a diamond tester?

No. Standard diamond testers measure thermal conductivity, and cubic zirconia does not conduct heat the way diamond does, so it fails the test. Moissanite, by contrast, can pass a thermal tester and requires a separate electrical conductivity test to identify.

Is cubic zirconia good for an engagement ring?

For long-term daily wear, no. CZ scratches, clouds, and loses sparkle far faster than diamond. It can work as a temporary stone or for a fashion-focused ring, but most buyers who choose CZ for an engagement ring end up replacing the stone.

Why does cubic zirconia look so rainbow-colored?

CZ has higher dispersion than diamond, which causes more colored light to scatter from the stone. That extra fire reads as rainbow flashes, which is why CZ often looks more sparkly but also less natural than a diamond.

How much cheaper is cubic zirconia than diamond?

A one-carat CZ retails for roughly $20 to $40. A one-carat natural diamond of decent quality starts around $2,000. Lab-grown diamonds fall in between, typically $400 to $1,500 per carat depending on grade.

Can a jeweler tell the difference between diamond and cubic zirconia?

Yes. A jeweler can identify cubic zirconia in seconds using a thermal conductivity tester, magnification, or a specific gravity check. If you have a stone you want identified, a quick visit settles it.

Diamond vs Cubic Zirconia: Key Differences Explained

J M Scully
Jun 5
8 min read

Diamond is a natural carbon crystal that scores a 10 on the Mohs hardness scale and holds long-term resale value. Cubic zirconia is a synthetic stone made from zirconium dioxide that scores 8 to 8.5, costs a fraction of the price, and is classified as a diamond simulant rather than a real diamond. Both stones can look similar at a glance, but they differ in chemistry, sparkle pattern, durability, and what they are worth ten years from now.

If you are choosing a stone for an engagement ring, restoring an heirloom, or just want to know what is actually in the jewelry you already own, the differences below are the ones that matter.

Quick Answer: Diamond vs Cubic Zirconia

Composition: Diamond is pure carbon. Cubic zirconia is zirconium dioxide.
Origin: Diamonds are formed naturally or grown in a lab. Cubic zirconia is always lab-made.
Mohs hardness: Diamond is a 10. Cubic zirconia is 8 to 8.5.
Refractive index: Diamond is about 2.42. Cubic zirconia is about 2.15 to 2.18.
Dispersion (fire): Diamond is 0.044. Cubic zirconia is 0.058 to 0.066, which is why CZ throws more rainbow flashes.
Weight at the same size: Cubic zirconia weighs roughly 1.7 times more than a diamond of the same dimensions.
Resale value: Diamonds retain measurable resale value. Cubic zirconia has essentially none.
Best uses: Diamond for engagement rings, heirloom jewelry, and pieces meant to last decades. Cubic zirconia for fashion jewelry, travel pieces, and temporary settings.

What Is a Diamond?

A diamond is a crystal made of carbon atoms locked into a tetrahedral lattice. Natural diamonds form deep in the earth under heat and pressure over billions of years. Lab-grown diamonds are made in a controlled environment that recreates those same conditions, and they share the same chemical and physical properties as natural diamonds.

Both natural and lab-grown diamonds are real diamonds. They are graded on the same four Cs (cut, color, clarity, and carat weight), and a well-cut diamond produces a balanced mix of white brilliance, colored fire, and scintillation that no simulant fully replicates.

Ready for “the real thing”? Shop Diamond Stud Earrings

What Is Cubic Zirconia?

Cubic zirconia is the cubic crystalline form of zirconium dioxide. It was first synthesized for jewelry use in the 1970s as an affordable diamond alternative, and by the mid-1980s, it was being produced by the tens of millions of carats for fashion jewelry (Wikipedia).

Because it is grown in a lab from a different material than diamond, cubic zirconia is classified as a diamond simulant. It is engineered to look like a diamond, not to be one. It is not a lab-grown diamond, and it does not have a diamond's carbon structure, hardness, or optical performance.

Sparkle, Brilliance, and Fire

The way each stone handles light is one of the easiest visual tells, once you know what to look for.

Diamond has a higher refractive index, which means light bends more sharply when it enters the stone and reflects as crisp white flashes. This is what gemologists call brilliance. Diamond also produces a measured amount of colored light, called fire, but the white-to-rainbow ratio stays balanced.

Cubic zirconia has a lower refractive index but higher dispersion. The result is more rainbow flashes and less white light. Under natural light, a cubic zirconia stone often shows what jewelers call the rainbow effect, where the stone throws so much colored light that it looks slightly artificial to a trained eye (Diamonds Pro). Some buyers love that extra fire. Others find it reads as a costume.

Diamond: Sharp white flashes with some rainbow fire.
Cubic zirconia: Heavy rainbow flashes with less white light, sometimes described as a "disco ball" effect.
Practical takeaway: Under a single small light, a diamond produces distinct white and colored flashes in roughly equal measure. CZ produces a more diffuse, color-heavy sparkle.

Durability and Everyday Wear

This is where the gap is widest, and where it actually affects what you should buy.

Diamond is the hardest natural material known and scores a perfect 10 on the Mohs hardness scale. Cubic zirconia scores 8 to 8.5, which sounds close on paper but is not. The Mohs scale is not linear, and diamond is several times harder than CZ in real-world wear.

What that means for jewelry:

Scratching: Diamond can only be scratched by another diamond. Cubic zirconia can be scratched by topaz, sapphire, ruby, hardened steel, and the abrasives in some cleaning products.
Surface wear: A CZ stone worn daily will collect fine scratches and lose sharpness on its facet edges within a few years. Diamond facets stay crisp for generations.
Clouding: Cubic zirconia is more porous than diamond and absorbs oils, lotions, and residue. Over time, the surface becomes hazy or cloudy, which is why many CZ rings dull within two to three years of daily wear.
Chipping: Diamond is harder but more brittle along its cleavage plane. A sharp impact at the wrong angle can chip a diamond, while the same impact on a CZ tends to leave a small dent rather than a chip (Curated Sense).

If a stone needs to survive thirty years of dishes, hand washing, and the occasional door frame, a diamond is the safer bet. If a stone needs to look good for a season, cubic zirconia is fine.

Cost, Value, and Resale

A one-carat round cubic zirconia retails for roughly $20 to $40. A one-carat round diamond of decent quality starts around $2,000 and climbs from there depending on cut, color, and clarity grade.

That price gap is real, but the resale picture is where the gap widens further. Diamonds retain meaningful resale value, especially when they come with a recognized lab grading report and a record of care. They are not a guaranteed investment, and the gap between retail price and resale price is often significant, but a quality diamond is a durable store of value over time.

Cubic zirconia has essentially no resale market. Once a CZ stone is worn, it is worth what someone will pay for a costume piece, which is rarely more than a few dollars.

The honest framing:

Diamond: Holds long-term value. Can be passed down, reset, or sold, though not at retail price.
Cubic zirconia: No meaningful resale value. Treat the purchase as the full lifetime cost.

This matters most for engagement rings and heirloom pieces, where the stone is expected to outlast the setting and the trend cycle.

Is Cubic Zirconia a Lab-Grown Diamond?

No. This is the single most common piece of confusion buyers carry into a jewelry store, and the answer is straightforward.

A lab-grown diamond is a real diamond. Its chemistry, hardness, and optical properties are identical to those of a natural diamond. The only difference is where it was made (Brides).
Cubic zirconia is not a diamond at all. It is zirconium dioxide grown in a lab to look like a diamond. It is softer, less brilliant, and has none of diamond's carbon structure.
If someone tells you a stone is "lab-made," ask whether it is a lab-grown diamond or a cubic zirconia. The two are not the same, and the price difference between them is enormous.

How to Tell Diamond from Cubic Zirconia

A jeweler with the right equipment can identify a cubic zirconia in under a minute. At home, the visual checks are less reliable, but a few hold up.

Professional methods (most reliable):

Thermal conductivity test. A diamond tester pen measures how fast the stone conducts heat. Diamond conducts heat very efficiently. Cubic zirconia does not, so it fails the test (Do Amore). Note: moissanite passes a thermal tester and requires a separate electrical conductivity test to distinguish from diamond.
Magnification under a loupe or microscope. Diamond facet edges are sharp and clean. CZ facets are slightly rounded and softer at the edges. Natural diamonds also often show small internal inclusions that CZ lacks.
Specific gravity test. Cubic zirconia is roughly 1.7 times heavier than diamond at the same size, so a stone that feels too heavy for its dimensions is a CZ.

At-home checks (less reliable):

Sparkle pattern. Hold the stone under a single light. Diamond shows sharp white flashes. CZ shows mostly a rainbow color.
Fog test. Breathe on the stone. Diamond clears almost immediately because it dissipates heat fast. CZ stays fogged for a few seconds.
Newspaper test. Place a loose stone face down on the printed text. You can usually read through a CZ. You cannot read through a well-cut diamond.

None of the at-home tests is conclusive. If you have a stone you want identified for certain, bring it to a jeweler. A five-minute visit settles it.

Cubic Zirconia and Diamond Rings: Which Should You Choose?

The decision is not "which stone is better." The right answer depends on the role the jewelry is supposed to play.

Choose diamond for:

Engagement rings meant for daily wear
Wedding bands and anniversary jewelry
Heirloom pieces you plan to pass down
Any ring that needs to look the same in twenty years as it does today
Pieces where resale value matters

Choose cubic zirconia for:

Fashion jewelry and trend pieces
Travel rings (replacing a more valuable ring while on vacation)
Temporary engagement rings (proposal rings later reset with a diamond)
Budget-focused gifts where sparkle matters more than longevity
Stones for children or teens whose tastes will change

The most common mistake is putting a CZ in a setting that is meant to last and being disappointed when it clouds. The second most common is putting a diamond in a piece that will rarely be worn, which ties up money in a stone that sits in a drawer.

Caring for Each Stone

Both stones benefit from regular cleaning, but the upkeep curves are different.

Diamond care:

Clean every few weeks with warm water, mild dish soap, and a soft brush.
Have prongs and settings checked once a year by a jeweler.
Professional cleaning once or twice a year keeps the stone at its best.
Avoid wearing during heavy lifting, rock climbing, or anything that risks a sharp side impact.

Cubic zirconia care:

Clean weekly with warm soapy water. Skip ultrasonic cleaners, which can damage the stone over time.
Remove before showering, swimming, applying lotion, or using cleaning products.
Store separately from harder stones to avoid scratches.
Expect to replace the stone eventually. Most CZ pieces show visible wear within two to five years of daily use.

For long-term storage advice that applies to both, see best way to store jewelry.

How Diamond Compares to Other Diamond Alternatives

Cubic zirconia is one option among several. The two that it gets confused with most often:

Moissanite. Scores 9.25 on the Mohs scale, harder than CZ and close to diamond. Its dispersion is even higher than CZ, so it throws strong rainbow flashes. Moissanite passes a standard thermal diamond tester, which CZ does not.
Lab-grown diamond. A real diamond made in a lab. Same hardness, same optics, same chemistry as a natural diamond, at a lower price.

If sparkle and price are the priority, moissanite usually outperforms CZ in both durability and resale. If the goal is a real diamond at a lower cost, a lab-grown diamond is the right category, not cubic zirconia.

Expert Diamond and Gemstone Guidance in Holden, MA

If you have a stone you want identified, an heirloom you want appraised, or you are weighing a diamond against an alternative for an engagement ring, working with an experienced jeweler removes the guesswork. At JM Scully Jewelers, every consultation includes professional stone identification, honest guidance on what to choose for the role the jewelry will play, and one-on-one service from a local jeweler with decades of experience.

Whether you are exploring custom engagement rings, looking into diamond sales, or planning a heirloom restoration, stop by the shop in Holden or call to schedule a consultation.

For more on the diamond buying process, our guide on what to look for when selecting a diamond walks through the four Cs in plain language, and if you suspect a stone may not be what it was sold as, our piece on how to tell if a diamond is real covers the testing process in more detail.