Cubic zirconia, usually shortened to CZ, is a man-made crystal of zirconium oxide that has been the most common diamond imitation since the late 1970s. It looks very much like diamond at a glance and shows even more colored fire. CZ is softer, much heavier and far less able to conduct heat than diamond, and those differences make it easy for a jeweler to identify.
What cubic zirconia is
CZ is zirconium dioxide, ZrO2, grown in a cubic crystal structure. Pure zirconium oxide is not cubic at room temperature: it changes to a different structure as it cools and cracks apart. Adding a stabilizer, usually yttrium oxide and sometimes calcium oxide, keeps the cubic structure stable down to room temperature.
CZ is a simulant, not a lab-grown diamond. It imitates diamond's appearance but has a different chemistry and structure. It should also not be confused with zircon, a natural zirconium silicate gem with a long history of its own, described on the zircon page.
The cubic form of zirconium oxide does occur in nature, but only as tiny grains in a few rocks. Every CZ used in jewelry is manufactured.
How cubic zirconia is made
Scientists in the Soviet Union, working at the Lebedev Physical Institute in Moscow, developed a practical way to grow large, clear crystals in the early 1970s. Their method, called skull melting, solved a hard problem: zirconium oxide melts at about 2,700 degrees Celsius, too hot for any crucible.
In skull melting, zirconium oxide powder and stabilizer are packed into a cup made of water-cooled copper tubes. Radio-frequency energy heats the powder from the inside and melts it, while the outer layer stays cool and solid against the copper. That solid crust, the "skull," acts as its own crucible. When the power is slowly reduced, columnar crystals grow upward through the melt. After cooling, the skull is broken open and the crystals are sorted and cut.
Commercial production spread in the mid to late 1970s, and CZ quickly displaced earlier simulants such as strontium titanate, synthetic rutile and yttrium aluminum garnet (YAG). By the 1980s it had become the standard affordable diamond look-alike.
Manufacturers can add trace elements to produce CZ in almost any color, including pink, yellow, green, blue and purple.
How CZ compares with diamond
| Property | Diamond | Cubic zirconia |
|---|---|---|
| Chemistry | Carbon | Zirconium oxide with stabilizer |
| Mohs hardness | 10 | 8 to 8.5 |
| Refractive index | 2.417 | About 2.15 to 2.18 |
| Dispersion | 0.044 | About 0.058 to 0.066 |
| Specific gravity | 3.52 | 5.6 to 6.0 |
| Thermal conductivity | Very high | Very low |
Dispersion and fire
CZ disperses white light more strongly than diamond, so it shows more flashes of spectral color. Under spotlights, a CZ can look more colorful than a diamond of the same size. Experienced eyes often read that extra fire as a sign of an imitation. The diamond optical properties page explains dispersion and brilliance.
Lower refractive index
CZ bends light less than diamond does. With a lower refractive index, light leaks out of a CZ more easily, so a CZ shows slightly less brilliance and a softer, less crisp pattern of light and dark facets than a well-cut diamond.
Weight
CZ is about 1.7 times as dense as diamond. A round CZ with the same diameter as a 1-carat diamond weighs roughly 1.6 to 1.75 carats. For that reason, CZ is often sold by millimeter size or by "diamond equivalent" weight, not by actual carat weight. A loose stone that seems too heavy for its size on a gem scale is a likely CZ.
Hardness and wear
At 8 to 8.5 on the Mohs hardness scale, CZ is harder than most gems but softer than diamond and corundum. Everyday dust contains quartz at Mohs 7, which cannot scratch CZ, but harder grit and contact with other gems can. Over years of wear, CZ facet edges tend to become rounded and scratched, and the surfaces pick up a dull look. A diamond's edges stay sharp far longer.
How to tell CZ from diamond
Several observations point toward CZ. None is foolproof on its own, but together they are usually conclusive.
- Thermal tester. Diamond conducts heat extremely well and CZ does not. A standard thermal diamond tester separates the two reliably.
- Weight. A loose CZ weighs much more than a diamond of the same dimensions.
- Fire. Strong, rainbow-colored flashes under spotlights suggest CZ.
- Wear. Rounded facet junctions and scratched facets on a ring that has been worn for some years suggest CZ.
- Girdle and facets. Diamond girdles are often frosted or finely faceted and may carry a laser inscription. CZ girdles are usually polished smooth, and facet junctions under a loupe are often less sharp.
- Fluorescence. Many CZs glow yellowish or greenish yellow under shortwave ultraviolet light, a different reaction from most diamonds.
- Newsprint. Lines of text seen through a loose, table-down CZ may be readable or partly readable, while a well-cut round diamond usually blurs them, though this test fails with many fancy shapes and poorly cut diamonds.
The guide on how to tell if a diamond is real explains which home tests work and why professional testing settles the question.
Coated and branded CZ
Some sellers coat CZ with a thin layer of diamond-like carbon or other material and market it under trade names. These coatings may slightly change surface properties, but they do not make CZ a diamond, and a thin coating can wear off. Labs can identify coated CZ.
CZ is also used in high-volume costume and fashion jewelry and as a stand-in for a diamond while a fine piece is being designed or repaired. Its low cost makes it popular for travel jewelry.
Care
CZ resists most household chemicals and is safe to clean in warm, soapy water. Scratches and abrasion are the main concern, so store CZ jewelry separately from diamonds and other hard gems. Ultrasonic cleaners are generally safe for CZ itself, though the setting and any coating should be considered. The jewelry cleaning guide covers safe methods.
CZ among other simulants
Diamond simulants have a long history, from leaded glass "paste" in the 18th century to the synthetic crystals of the 20th. The glass in jewelry page covers older imitations, and moissanite became CZ's main rival among modern simulants after the late 1990s.
Frequently asked questions
Is cubic zirconia a real diamond?
No. It is zirconium oxide, a different material made in a lab. It imitates diamond's appearance but has different chemistry and properties.
Does cubic zirconia get cloudy?
CZ itself does not turn cloudy, but its surface scratches more easily than diamond and can collect grease and soap film. Cleaning restores most of its look, while scratches stay.
Will cubic zirconia pass a diamond tester?
No. A standard thermal diamond tester identifies CZ as not diamond, because CZ conducts heat poorly.
Is cubic zirconia the same as zircon?
No. Zircon is a natural zirconium silicate mineral. Cubic zirconia is a synthetic zirconium oxide.