Zircon

Zircon has a naming problem it did nothing to deserve. It is a natural mineral, one of the oldest on earth, with a brilliance close to diamond's and a long history as a gem in its own right. Cubic zirconia is a synthetic manufactured in a laboratory since the 1970s. The two share a syllable and nothing else, and the confusion has cost zircon most of its reputation.

What it actually offers is unusual: a refractive index and dispersion close enough to diamond's that colourless zircon was the diamond substitute of choice for centuries, plus a range of colours of which the blue is the one the modern market means when it says the word. It is also a December birthstone, and the one of the three with no guide on this site until now.

This page covers what zircon is and is not, why almost all blue zircon is heated, the brittleness that decides how it should be set and stored, the sources, what it costs, and the strange fact that the same mineral is how geologists date the planet.

What it is, and what it is not

Zircon is zirconium silicate, a natural crystal that forms in igneous and metamorphic rock and survives almost everything that happens to that rock afterwards. Cubic zirconia is zirconium oxide, grown in a furnace as a diamond imitation. They are different compounds with different structures, and the only connection is that both contain the element zirconium.

The distinction matters commercially because zircon is regularly dismissed as a fake by people who have confused it with the synthetic. Natural zircon has been cut and worn since antiquity; colourless stones from Sri Lanka were sold as Matura diamonds, and the reddish orange variety was the hyacinth or jacinth of the ancient world.

Two optical properties explain the diamond comparison. Zircon's refractive index runs to around 1.98, and its dispersion of 0.039 sits just below diamond's 0.044. In a good cut that produces a hard bright lustre and visible fire that no other natural colourless stone matched before synthetics existed.

The doubling, and how to tell it from anything else

Zircon is strongly doubly refractive. Look through the table of a stone of a carat or more with a loupe and the back facet edges appear doubled, printed slightly out of register. Diamond, cubic zirconia and glass are all singly refractive and show nothing of the kind.

That one observation separates zircon from all three of the stones it is most often confused with, and it is visible without equipment on anything but the smallest stone. It is the same test that identifies peridot, and for the same reason.

The colours, and why blue is heated

Zircon comes out of the ground in brown, yellow, orange, red, green and occasionally colourless. The blue the market wants is not one of those. It is produced by heating brown material from Cambodia and Thailand to around a thousand degrees in an oxygen-free atmosphere, which turns it a bright greenish blue that is stable under normal conditions.

Essentially all blue zircon has been heated, and most colourless zircon has been too, since heating in air bleaches the brown. The treatment is permanent, universally practised and carries no price penalty, in the same way that heating is simply the normal state of tanzanite. It should still be stated, and any seller describing blue zircon as untreated is describing a stone that does not exist in commercial quantity.

Metamict zircon

Zircon incorporates traces of uranium and thorium as it grows, and over geological time the radiation from those elements breaks down the crystal structure from within. Heavily affected stones, called low or metamict zircon, are cloudy, greenish, softer and lower in refractive index than intact material. High zircon is the intact, bright, hard form that all the gem trade's numbers refer to.

The radioactivity involved is at trace level and not a concern for wearing. Its lasting consequence is the other way round: because zircon locks uranium in and keeps a record of its decay, it is the mineral geologists use to date rocks. Grains from the Jack Hills in Western Australia are about 4.4 billion years old, which makes zircon the oldest known material formed on this planet.

Judging a zircon

The facet edges deserve a specific look. Zircon is brittle, and a stone that has been stored loose with other stones, or worn hard in an exposed setting, develops a frosted look at the junctions where facets meet. The trade calls it paper wear. It is repairable by repolishing but it lowers the price of an unrepaired stone.

Where it comes from

Cambodia, from the Ratanakiri province, is the source of the brown material that becomes the blue the market knows, and Thailand produces and heats a great deal of it. Sri Lanka has produced zircon in every colour for centuries and is the historic source of the colourless stones. Myanmar, Tanzania, Vietnam, Nigeria and Australia all contribute.

Origin is not a price factor. Nobody pays a premium for a Cambodian stone over a Sri Lankan one; the market prices the colour and the cut. Judge the stone, not the label.

Hardness and everyday wear

High zircon is 7.5 on the Mohs scale, which is respectable, and low zircon can be as soft as 6. The number understates the practical problem, which is brittleness. Zircon has no cleavage but it chips readily at facet junctions, and it is sensitive to sudden temperature change.

For a ring that argues for a protective setting and against an exposed high solitaire, and it argues strongly against ultrasonic and steam cleaning, both of which can chip an edge or open a fracture. Our guide to settings covers the choices. Store it separately from other jewellery; a zircon rattling in a box with harder stones is how paper wear happens.

Heated blue zircon is stable in ordinary wear. Prolonged exposure to strong ultraviolet light has been reported to dull some stones, so it is not a stone to leave on a sunny windowsill, but nothing in daily wear approaches that.

What it costs

Colour first, then size, with blue setting the pace. In dollars per carat:

That makes zircon one of the more affordable ways to put a genuinely brilliant natural stone in a ring, and a far better buy than its reputation suggests. It also puts colourless zircon well below moissanite as a diamond alternative, at the cost of considerably less durability.

Meaning and symbolism

Zircon is a December birthstone alongside tanzanite and turquoise, and the December page sets the three together. It is the oldest of the three traditions by a wide margin: the hyacinth of antiquity was a zircon, and medieval lists of protective stones regularly include it.

The traditional associations are wisdom, honour and restful sleep, and the stone was worn as a guard against poisons and plague. As with all such traditions the evidence is thin, but zircon's is at least genuinely old rather than invented for a modern catalogue.

If you are choosing by tradition rather than by month, our astrology and gemstones guide sets out the zodiac schemes, which assign stones on a different basis.

Designing around a zircon

• Protect the edges. Brittleness rather than softness is the risk, and it shows at facet junctions. A bezel, a half-bezel or a well-covered prong setting all work. Our settings guide covers the range.
• Keep it low. Height exposes the crown edges to knocks, and the fire is visible from a low profile just as well.
• White metal keeps blue zircon crisp, and yellow gold warms the colourless and golden stones handsomely. Our metals guide covers the comparison.
• Tell the setter what it is. Standard workshop cleaning will chip this stone, and the moment to prevent that is before the ring is handed over.
• Do not buy it as a diamond substitute for daily wear. As a brilliant coloured stone it is excellent value; as a stand-in for diamond in a ring that never comes off, it will show wear that diamond would not.

A short buying checklist

• Confirm it is natural zircon and not cubic zirconia, in writing, and check for facet doubling through the table.
• Look at the facet junctions under magnification for abrasion, which is the characteristic wear.
• Ask whether it is heated; for blue the answer is yes and that is normal, but it should be stated.
• Check for cloudiness, which suggests low zircon rather than the bright high form the prices assume.
• Tilt for a window, which shows where weight was kept at the expense of the brilliance you are buying.
• For anything substantial, ask for a report from an independent laboratory naming the species.

Frequently asked questions

Is zircon the same as cubic zirconia? No. Zircon is a natural mineral, zirconium silicate, worn as a gem since antiquity. Cubic zirconia is a synthetic zirconium oxide made since the 1970s as a diamond imitation. They are unrelated.

Is blue zircon natural? The mineral is natural; the colour is produced by heating brown material. Essentially all blue zircon is heated, the treatment is permanent and standard, and it carries no price penalty.

Is zircon radioactive? It contains trace uranium and thorium, as many minerals do, at levels that are not a concern for wearing. The same traces are what let geologists use zircon to date rocks.

Is it hard enough for an engagement ring? High zircon is 7.5, but it is brittle and chips at facet edges. In a protective setting, worn with some care, it works; as an exposed daily solitaire it will show wear.

Why does it look like a diamond? Because its refractive index and dispersion are both close to diamond's. Colourless zircon was the standard diamond substitute before synthetics existed.

Why do the back facets look doubled? Strong double refraction, which splits light into two rays inside the stone. It is normal, it is diagnostic, and it is the quickest way to tell zircon from diamond, cubic zirconia or glass.

What is paper wear? Abrasion at the facet junctions from the stone knocking against harder stones or surfaces. It gives the edges a frosted look, it lowers the price, and repolishing removes it.

How do I clean it? Warm water, mild soap and a soft brush. Never an ultrasonic or steam cleaner, and never a sudden change of temperature.

To see how it sits against the rest of the coloured stones, the semi-precious stones guide compares the series side by side, and we are happy to answer questions directly (opens in new tab).