Zircon vs. Cubic Zirconia: How to Tell Them Apart

Two names, two materials: what zircon and cubic zirconia are

Zircon is zirconium silicate, ZrSiO₄, a tetragonal mineral that occurs as tiny grains in almost every granite and grows into cuttable crystals in river gravels. The name probably goes back to the Persian zargun, "gold-colored." It is a gemstone with a long history of its own: cut in Sri Lanka for centuries, worn by eighteenth- and nineteenth-century aristocrats as "jargoon" or "hyacinth," and around 1920 blue zircon from Indochina was one of the favorite blue stones of Art Deco jewelers.

Cubic zirconia, CZ for short, is cubic zirconium oxide, ZrO₂. It is not a mineral in any practical sense but a manufactured material; in nature the compound turns up only as microscopic grains. Scientists at the Lebedev Physical Institute in Moscow worked out how to grow it in gem quality in the 1970s, and the Soviets called it fianit. Because it can be grown colorless, clean and cheap and bends light almost as strongly as diamond, within a few years it became the world's most important diamond simulant. The two share the element zirconium and nothing else: not the chemistry, not the crystal system, not the origin, not the price.

Zircon vs. cubic zirconia: the numbers that make the difference

Property	Zircon (natural)	Cubic zirconia (lab-made)	Diamond (for comparison)
Chemistry	ZrSiO4, a silicate	ZrO2, an oxide	C, carbon
Crystal system	tetragonal	cubic	cubic
Mohs hardness	6.5 to 7.5	8 to 8.5	10
Specific gravity	3.9 to 4.7	5.6 to 6.0	3.52
Refractive index	1.92 to 1.98, strongly doubly refractive	2.15 to 2.18, singly refractive	2.42, singly refractive
Dispersion (fire)	0.039	0.060	0.044
Origin	Sri Lanka, Cambodia, Tanzania, Australia	laboratory, since the 1970s	mine or laboratory
Typical price	tens to low hundreds of dollars per carat	cents to a few dollars per stone	thousands per carat

The oldest material you will ever touch

Zircon has one property that makes it irreplaceable to geologists: when it crystallizes it takes up traces of uranium but essentially no lead. Every atom of lead found in a zircon today was produced by the decay of that uranium, and because the decay runs at a known rate, every zircon is a clock. Using this uranium-lead dating, a team led by Simon Wilde reported in Nature in 2001 that zircon grains from the Jack Hills in Western Australia are about 4.4 billion years old: older than any rock on Earth, formed only around 150 million years after the planet itself. Their chemistry even suggests there was liquid water on the surface that early.

The same uranium has a second consequence you should know about. Some zircons are weakly radioactive, and over millions of years that radiation damages their own crystal lattice. Gemologists therefore distinguish "high" zircon (intact, hard, brilliant) from "low" or metamict zircon (greenish, cloudy, softer, with a lower refractive index). For jewelry this is not a concern, since the dose is tiny, but it explains why a green zircon from Sri Lanka looks so different from a blue one from Cambodia. If you want to know which stones really do call for caution, our guide to crystals that cannot go in water is a good place to start.

Colors: hyacinth, jargoon, and the heated blue of December

Natural zircon is mostly brown, reddish or yellowish. Red-brown to orange zircon was called hyacinth for centuries, a name that appears in old lapidaries and in English Bible translations, though there it probably meant a different stone. Colorless and pale yellow zircon was called jargoon and served in the eighteenth century as a diamond substitute for people who could not afford one, the role cubic zirconia plays today.

The glowing blue that made zircon famous is almost always heated. Brown stones from Cambodia and Thailand are heated to around 1,700 to 1,800 °F in the absence of air and turn blue; heated with air, they turn colorless or golden. This has been standard in the trade for a hundred years and the color is generally stable. Untreated blue zircon is practically nonexistent; anyone offering you a "naturally blue" one either does not know or is not telling. It is this heated blue that sits on the American birthstone list for December, alongside turquoise and tanzanite. Our birthstones by month guide explains where that list came from and who changes it.

How to tell zircon, cubic zirconia and diamond apart

The good news: these three stones differ so much that simple tools separate them almost every time, as long as you know what to look for.

1. The doubling. Hold a 10x loupe over a faceted stone and look at an angle through the table toward the back facet edges. In zircon they appear clearly doubled: it has one of the strongest double refractions of any gemstone, far more obvious than ruby or sapphire. Cubic zirconia and diamond show single, crisp edges. This one look separates real zircon from both of the others.
2. The weight. Cubic zirconia is heavy, about 1.7 times as dense as diamond. A round brilliant CZ about a quarter inch (6.5 mm) across weighs roughly 1.7 carats; a diamond of the same size weighs about 1 carat. If a "diamond" is too heavy for its size, it is CZ. Zircon falls in between.
3. The edges. Zircon is brittle. On stones that have been worn, the facet edges are often chipped in tiny nicks, as if clipped with a fingernail; jewelers call it "paper wear." CZ has smooth, intact edges that slowly turn dull with age. Diamond stays sharp.
4. The fire. Cubic zirconia throws rainbow flashes, more than a diamond does. Too much. It is the "disco sparkle" that trained eyes read as fake at once. Zircon sparkles lively but calmer; a good blue zircon has more fire than a sapphire and is often mistaken for one.
5. The breath test. Fog the stone with your breath. On diamond the fog vanishes almost instantly because diamond conducts heat extremely well; on CZ and zircon it lingers for two or three seconds. That separates both from diamond, but not from each other; for that you need step 1. A handheld thermal diamond tester does the same job more reliably.
6. The price tag. A "diamond" for twenty dollars is CZ. A "zircon" for twenty dollars is also almost always CZ, because sellers mix up the names, on purpose or out of ignorance. A real faceted zircon in good color costs tens of dollars per carat and up.

Why the real stone suffers for the fake one

Zircon has a naming problem handed to it by a material decades younger than its career as a gem. Since cubic zirconia went on sale everywhere from mall kiosks to TV shopping channels, "zircon" sounds cheap to most people, and some sellers make it worse by listing CZ as "zircon." In the United States the Federal Trade Commission's Jewelry Guides (16 CFR Part 23) treat it as deceptive to use the name of a natural stone for a lab-made or imitation product without a clear qualifier, but online listings do it every day. The result: the one gemstone that combines genuine diamond-like fire with a natural origin is cheaper today than a century ago and rarely sits in crystal shops, because nobody asks for it. For you that is an opportunity. For the stone it is a misunderstanding.

What crystal lore attributes to zircon

Crystal lore knows zircon mainly as hyacinth, and under that name it appears in medieval lapidaries as a protection against plague and lightning and as a stone that brings sleep and takes away sorrow. Hildegard of Bingen discusses the hyacinth in her Physica, for the eyes and the heart. In modern crystal books, zircon is described as a stone for processing loss and letting go, one that leads out of grief back into action. I like to read that against its geology: a mineral that has survived 4.4 billion years of everything the planet did to it, as a stone for people who are getting through something. Whether a silicate can do that, I do not know. That the image holds, I do.

Cubic zirconia, for completeness, has no tradition and no attributions: it was born in a lab in the 1970s, and anyone ascribing effects to it made them up recently. As a jewelry stone it is still perfectly respectable. A CZ sold as a CZ is a clean, hard, brilliant stone for very little money. The only dishonest thing is the wrong label.

Care and wear

Zircon, at hardness 6.5 to 7.5, is fine for everyday wear but brittle: not a stone for a ring on the hand you work with, but good in earrings, pendants and rings you take off for the gym. Clean it with lukewarm water and a soft brush; ultrasonic cleaners are acceptable for intact stones but best avoided for metamict ones. It tolerates heat, but strong heat can change a heated color. Avoid weeks of direct sun: some heated blues slowly turn brownish under UV and recover in the dark. Cubic zirconia is harder and tough, but it turns dull over the years as oil and mineral deposits settle into microscopic scratches; a soak in warm water with dish soap brings the sparkle back. For how hardness decides what goes into a ring, see our Mohs hardness guide.

Quiz: which stone are you holding?

From the practice

A gem cutter I asked about zircon has one sentence he tells every apprentice: "You cut zircon like glass that hates you." The stone is brittle, the edges chip, and its strong double refraction makes cutting demanding, because every facet appears doubled unless the table sits exactly right relative to the optical axis. Good zircons are therefore rarely well cut, and a well-cut blue zircon, he says, is the most underrated stone in the trade.

A goldsmith who works with cubic zirconia every day is more relaxed about it: "I set CZ all the time and I am not ashamed of it. I am ashamed of the people who hide it." Her customers know what they are buying, and they come back. The problem, she says, was never the stone. It was the label. If you want to go further into spotting dyed, heated and relabeled stones in general, our guide to real vs. treated crystals continues from here.

What we do not promise

We do not claim that zircon lifts grief, brings sleep or protects against anything. Those are attributions from lapidaries and crystal lore, not demonstrated effects, and zirconium silicate has no mechanism to do any of it. We do not claim you can identify every stone without a refractometer: the loupe reliably picks out zircon, but separating CZ from other singly refractive lab stones such as moissanite simulants or synthetic spinel takes more. And we do not claim cubic zirconia is inferior. It is a good stone with an unfair reputation, just as zircon is a good stone with a borrowed bad one.

In practice: step by step

1. Read the label literally. "Zircon" and "zirconia" are two materials. If it says "cubic zirconia," "CZ" or "simulated," it is lab-made; if it says "zircon" on a stone under ten dollars, ask what is meant.
2. Buy zircon from a gem dealer, not a crystal shop. Crystal shops rarely have it, and when they do it is often a metamict tumbled stone. The beautiful one is faceted: blue, honey or red.
3. Check for doubling. Loupe, angle through the table, look at the back edges. Doubled means zircon. That is the one move worth learning.
4. Wear zircon protected. Earrings, pendants, evening rings. Not on the working hand, not at the gym. The edges chip.
5. Take CZ for what it is. Sparkle for little money, named honestly. Our spiritual jewelry collection states what is set in each piece; that is the only rule that counts.

Frequently asked questions

Is zircon a real gemstone?

Yes. Zircon (ZrSiO₄) is a natural mineral with a Mohs hardness of 6.5 to 7.5 that has been cut as a gemstone for centuries, and the oldest known crystals on Earth are zircons from the Jack Hills in Western Australia, about 4.4 billion years old. It has nothing to do with lab-made cubic zirconia.

What is the difference between zircon and cubic zirconia?

Zircon is a natural silicate mineral; cubic zirconia is zirconium oxide grown in a lab since the 1970s. Zircon is strongly doubly refractive, so its back facet edges look doubled under a loupe; CZ is singly refractive and much heavier. They share only the element zirconium and a similar name.

How can you tell if a stone is cubic zirconia?

By its weight (about 1.7 times heavier than a diamond of the same size), its exaggerated rainbow fire, single crisp edges under a loupe, and its price. When you breathe on it, the fog lingers for a few seconds; on a diamond it vanishes almost at once.

Is blue zircon natural?

The stone is natural, the color almost never is. Blue zircon is produced by heating brown zircon without air, a standard practice since about the 1920s, and the color is generally stable. Practically every blue zircon on the market has been heated.

Is zircon radioactive?

Some zircons contain traces of uranium and thorium and are weakly radioactive; over millions of years this can damage the crystal (metamict zircon). For jewelry the dose is considered negligible. Heavily metamict, cloudy stones are softer and less brilliant.

Is zircon a December birthstone?

Yes. Blue zircon is on the modern American birthstone list for December, together with turquoise and tanzanite. Crystal lore also calls zircon, historically known as hyacinth, a stone of letting go; that is tradition, not a demonstrated effect.