Gem library
Zircon
The oldest mineral on Earth
Zircon is a gem unfairly devalued by the confusion of its name with synthetic cubic zirconia. It is in fact an ancient natural stone, very dense and very bright, whose dispersion approaches that of diamond — and the oldest mineral ever dated on Earth.
- Hardness (Mohs)
- 6 – 7.5
- Density
- 3.90 – 4.73
- Crystal system
- Tetragonal
- Refractive index
- 1.810 – 2.024
- Cleavage
- Imperfect on {110}
Mineralogical identity
- Family
- Zircon — nesosilicate group
- Species
- Zircon
- Variety
- Starlite (heated blue), hyacinth, jargoon
- Chemical formula
- ZrSiO₄
- Crystal system
- Tetragonal
- Classification
- Semi-precious stone
- Rarity
- Common
- Indicative price
- €10 – 150/ct
Etymology
From the Persian zargun, «gold-coloured», via Arabic zarqūn. «Jargoon» once denoted the yellow zircons of Ceylon, and «hyacinth» the red ones.
Physical & optical properties
- Hardness (Mohs)
- 6 – 7.5
- Density
- 3.90 – 4.73
- Refractive index
- 1.810 – 2.024
- Birefringence
- 0.000 – 0.059
- Cleavage
- Imperfect on {110}
- Fracture
- Conchoidal, brittle in metamict stones
- Lustre
- Subadamantine to vitreous
- Diaphaneity
- Transparent
- UV fluorescence
- Weak
- Pleochroism
- Weak
- Colours
- blue, colourless, red, brown, yellow, green
Formation, geology & history
Zircon, zirconia: clearing it up
This must be stated plainly, because the conflation costs zircon dearly. Zircon is a natural mineral, a zirconium silicate, crystallising in granites for billions of years. Cubic zirconia — «CZ» — is an entirely synthetic zirconium oxide, produced in laboratories since 1976 as a diamond imitation and sold for a few euros a carat. The two share only zirconium and a syllable. A good natural blue zircon is worth a hundred times a zirconia, and the loose vocabulary of sellers sustains a misunderstanding of which the natural mineral is the victim.
The dean of minerals
The zircons of the Jack Hills in Western Australia have been dated at 4.4 billion years: they are the oldest known fragments of Earth's crust, older than any surviving rock. Their secret is chemical robustness: zircon survives erosion, transport, burial, and even the partial remelting of its host rock. It traps uranium, which decays to lead at a known rate, making it geology's absolute clock. Almost everything we know about the age of continents, the great extinctions and the building of mountain ranges has been read in zircon crystals.
Metamictisation, heating and colours
That same internal radioactivity eventually damages the crystal: zircons rich in uranium and thorium see their lattice break down over time. These are called metamict or «low type» — softer, less dense, greener and cloudier. Intact «high type» zircons reach a density of 4.7 and near-adamantine brilliance. Heating to 900-1000 °C partly restores the lattice and produces the market's colours: Cambodia's famous sky-blue «starlite», the colourless material that long served as a diamond substitute, the golden and the red hyacinth. This treatment is universal and generally stable, though some blues can shift under very intense light.
How to recognise it
- Doubled edgesIn non-metamict zircons, birefringence of 0.059 visibly doubles the back edges seen through the table with a loupe. Cubic zirconia, being isotropic, never does this.
- Weight in the handAt 4.0 to 4.7, zircon is one of the densest gems. A ten-millimetre stone weighs nearly twice as much as quartz of the same size: the sensation is immediate and reliable.
- Girdle wearZircon is brittle: older stones almost always show small chips and a nicked girdle, particularly on fine cuts. That characteristic flaw distinguishes it from flawless synthetics.
Imitations, treatments & confusions
- Cubic zirconia sold as zirconThe commonest ambiguity, often unintentional. Zirconia is isotropic — no doubling — denser still (5.7), and its excessive dispersion gives gaudier fire. Always ask the seller for the exact word.
- Glass and «Ceylon diamond»Old trade names — jargoon, Matura diamond — once meant colourless zircons, but today serve to move glass. Round bubbles, low density, soft edges.
- Blue topaz and aquamarineStarlite blue is close to treated topaz blue. Topaz is lighter (3.5), shows no doubled edges, and has clean basal cleavage. Weight in the hand separates them easily.
Deposits & origins
Cambodia (Ratanakiri — the source of starlite blue), Sri Lanka (the historic deposit, all colours), Myanmar, Tanzania, Nigeria, Madagascar, Australia (Jack Hills, the oldest crystals known), Vietnam, Thailand, France (Puy-de-Dôme, Massif Central)
Care & precautions
- Warm soapy water and a soft brush, without scrubbing the edges: zircon is brittle even at hardness 7.
- Avoid ultrasonic cleaning, which worsens micro-chipping on the girdle, especially on older or metamict stones.
- Limit prolonged exposure to very strong light: a few heated blues can change over time.
- Store separately and choose a setting that protects the girdle — that is where zircon suffers.
Traditions & crystal healing
- Chakras
- Root · sacral · crown
- Zodiac signs
- Sagittarius · Capricorn
- Element
- Fire · Earth
- Month
- December
Zircon is one of the oldest gems actually worn: medieval treatises call it a guarantor of sleep, a shield against plague and a token of wisdom. Red hyacinth appears in biblical lapidaries. Modern crystal healing treats it as a stone of grounding and purification, linked to both root and crown — a duality that revives the old idea of a stone joining earth to sky.
The virtues described here belong to tradition and cultural heritage. They are in no way medical advice and never replace a consultation.
Frequently asked questions
Is zircon a fake stone?
Absolutely not. Zircon is an ancient and estimable natural mineral; it is cubic zirconia, entirely synthetic, that is a diamond imitation. The vocabulary confusion has done great harm to natural zircon's reputation.
Is zircon's blue natural?
Cambodia's starlite blue results from high-temperature heating of brown crystals, a universal and generally stable practice. Naturally blue zircons are exceptional. The treatment must be disclosed, but it takes nothing away from the stone being natural.
Why does my zircon have small chips on the edge?
Because zircon is brittle: its toughness is low despite decent hardness. The sharp edges of the girdle chip over time. A covering setting and removing the jewel for manual work limit the problem.