What Is Cubic Zirconia (CZ)?

Cubic zirconia, usually shortened to CZ, is zirconium dioxide grown as a clear crystal in a factory. A small amount of a stabilizer, usually yttrium or calcium oxide, keeps the crystal in its cubic form so it stays clear and hard. Mass production for jewelry started in the 1970s, and within a few years CZ became the standard diamond imitation.

CZ is a simulant: it looks like diamond but is a different material. That separates it from a lab-grown diamond, which is real diamond made in a lab, and from moissanite, which is a different, much harder gem. The terms get mixed up constantly online. Our jewelry glossary sorts out the vocabulary.

Zirconium dioxide does occur in nature as a rare mineral, but only in tiny grains, never in gem sizes. Gem CZ is entirely man-made. Researchers in the Soviet Union developed a practical way to grow large clear crystals in the early 1970s using a technique called skull melting, in which the powder is melted inside a shell of its own unmelted material, since no ordinary crucible survives the heat. That method made CZ cheap enough to sell by the millions.

Because it is grown from pure chemicals, CZ has no natural inclusions and can be made in any color by adding small amounts of other elements. That flawless look is one of the clues that a stone is not diamond: almost every real diamond has some tiny inclusion under magnification.

Property	Cubic zirconia	Moissanite	Diamond
Material	Zirconium dioxide	Silicon carbide	Carbon
Mohs hardness	8 to 8.5	9.25	10
Refractive index	About 2.15 to 2.18	About 2.65 to 2.69	About 2.42
Density	About 5.6 to 6.0 g/cm3	About 3.2 g/cm3	About 3.5 g/cm3
Heat conductivity	Low	High	Very high
Basic thermal tester	Fails	Often passes	Passes
Look after years of wear	Scratched, cloudy, rounded edges	Holds polish	Holds polish
Resale value	Essentially none	Low	Lab: low. Natural: some

The density line is a quiet giveaway. CZ is roughly 1.7 times as heavy as diamond for the same size, so a loose 1 carat-sized CZ weighs well over a carat. On a scale, that is easy to spot. For a closer look at the two simulants, see CZ vs moissanite.

A Mohs rating of 8 to 8.5 sounds high, and it is harder than glass and quartz. The problem is everyday dust, which contains quartz at about 7, and constant contact with metal, counters and other jewelry. Over months, CZ facet edges round off and the table picks up fine scratches. Once scratched, the stone catches grease and dirt, which is why worn CZ looks cloudy and dull.

CZ is also brittle. It can chip at the girdle or corners from a knock that a diamond would shrug off. In a chain where every link is set with CZ, a few chipped or dull stones are usually the first sign of age.

Time worn daily	Typical CZ condition
New	Bright, sharp, hard to tell from diamond at a glance
Months	Fine scratches on the top facet, edges start to soften
A year or more	Visible haze, rounded facet edges, less sparkle

These are general wear patterns, not a fixed schedule. How fast a stone dulls depends on how often it is worn and how it is stored.

Most low-priced "iced out" chains, pendants, watches and earrings sold online use CZ set in brass, steel or sterling silver, often with gold plating. That is why a fully iced chain can be sold for under $200: neither the stones nor the metal are precious. There is nothing wrong with costume jewelry if you know that is what you are buying. The trouble starts when a listing uses words like "VVS", "real ice" or "diamond" for CZ.

The base metal matters as much as the stone. Plated brass and steel can wear through and react with skin; our page on why jewelry turns skin green explains why, and hallmarks and gold stamps covers how to read the metal markings.

CZ fools people for a reason. Its refractive index of about 2.15 to 2.18 is high, close enough to diamond's 2.42 that a new, clean CZ throws a lot of light. Its dispersion, the rainbow effect, is actually a little stronger than diamond's, so it can look very lively under store lights. On day one, in a display case, a well-cut CZ and a diamond of the same size can look similar to most shoppers.

The differences show with time and close inspection: CZ scratches and clouds, looks a bit glassy, and is unusually heavy. That is why a fresh piece in a photo or on a counter proves little, and why the receipt and a test matter more than first impressions.

1. Look at the edges. A 10x loupe shows rounded or nicked facet edges on worn CZ. Diamond edges stay crisp.
2. Watch the light. CZ often shows a slightly more colorful, glassier sparkle and can look too perfect, with no natural inclusions at all.
3. Use a thermal tester. CZ carries heat slowly and fails even the basic pen tester. This is the one test that is quick and reliable for CZ.
4. Weigh a loose stone. CZ is much heavier than diamond for the same measurements.
5. Check the metal. CZ jewelry is often in plated base metal; a missing or odd karat stamp is a clue.

The full testing guide, including moissanite, is in how to spot fake diamonds. For watches set with small stones, see what a bust down watch is.

CZ is often sold with grades like "AAA" or "5A". These are seller labels, not a standard set by any lab, so one company's 5A can differ from another's. Some CZ is coated with a thin film to reduce the rainbow look or add color; coatings can wear off with cleaning.

Term in a listing	What it usually means
CZ, cubic zirconia	Zirconium dioxide simulant
Simulated diamond, diamond simulant	Usually CZ, sometimes another simulant
AAA, 5A, 8A grade	Seller's own quality label, no lab standard
Lab-created diamond, lab-grown diamond	Real diamond, not CZ
Iced, flooded, bust down	Describes the look only; says nothing about stone type

The stones themselves have essentially no resale value; CZ costs very little to make. If the setting is solid gold, the gold has value by weight and karat. If the setting is plated base metal, the piece is worth little beyond its use as costume jewelry.

That is why the words on the receipt matter. "Carat" (ct) measures stone weight and "karat" (K) measures gold purity; a listing that says "10 carat" when it means 10K gold, or that never states the metal at all, is a warning sign. Carat vs karat explains the difference.

The appeal of CZ is price. A CZ stone costs very little, so a fully stone-set chain can sell for under $200. Real diamond, even lab-grown, costs far more. For comparison, real lab diamond pieces in our catalog:

Piece	Stone	Price
Round Lab Diamond Bracelet 7.49 ct, 10K White Gold	7.49 ct lab-grown diamond	$2,200
Round Lab Diamond Necklace 9.18 ct, 14K White Gold	9.18 ct lab-grown diamond	$5,300
Round Lab Diamond Ring 1.5 ct, 14K White Gold	1.5 ct lab-grown diamond	$2,000

If a piece promising similar carat weights costs a small fraction of these prices, it is almost certainly CZ or another simulant. The price itself is the first test.

The FTC Jewelry Guides say a seller should not call a stone a diamond unless it is one, and must clearly disclose lab-grown diamonds as lab-grown. You can use that to protect yourself. Before paying, ask for a receipt that lists:

• The stone type in plain words: natural diamond, lab-grown diamond, moissanite or cubic zirconia.
• Total carat weight of the stones, if they are diamond.
• The metal and karat, such as 10K or 14K gold, or sterling silver.
• Whether the metal is solid, plated or filled.

A seller of real diamonds has no reason to avoid writing "diamond". Every piece on this site is listed with its stone, from Round Lab Diamond Ring 1.5 ct, 14K White Gold at $2,000 to the Spark Diamond Solitaire Earrings at $740, which are 0.30 carats of diamond in 14K gold. Clasps and construction also separate real pieces from costume ones; chain clasp types shows what to look for.

If you own CZ pieces, they last longer with a little care. Store them apart from diamond and gold pieces, which can scratch them. Clean with warm soapy water and a soft brush, and avoid harsh chemicals that can attack plating on the metal underneath. Take them off for the gym, pool and beach. Once a CZ stone is badly scratched or cloudy, it is usually cheaper to replace than to repolish.

CZ is an honest material when it is sold as CZ: cheap, bright and fine for fashion pieces you will replace. It is never a diamond, it does not hold value, and it wears out faster than real gems. If a piece will be worn every day or given as a lasting gift, real diamond, natural or lab-grown, is the stone that will still look the same years from now.