Zircon
Zircon is Zr(SiO4): hardness 7.5, density 4.6 to 4.7, radioactive when metamict, per the sheet. A crystalline 20.00 g piece reads 15.65 to 15.74 g in water.
Zircon is a zirconium silicate, Zr(SiO4): tetragonal, hardness 7.5, density 4.6 to 4.7, with refractive indices of 1.925 to 2.015 and very strong dispersion. It is grandfathered on the IMA list, 1789, Sri Lanka. The Handbook sheet records it as radioactive when metamict. It is not cubic zirconia, which is laboratory-made ZrO2.
In short
- Two ways of writing one formula. The September 2026 IMA-CNMNC Master List gives Zr(SiO4), status G, 1789, with Sri Lanka as the country of first discovery; the Handbook of Mineralogy sheet, dated 2001, writes ZrSiO4. On standard atomic weights the formula is 49.77% zirconium, 67.22% ZrO2 and 32.78% SiO2 by mass.
- The hafnium check. The sheet’s only analysis, from Sitarampuram, India, carries 1.15% HfO2. Worked per silicon atom it gives 0.990 zirconium and 0.010 hafnium, 99 to 1, the formula the sheet prints beneath it. The sheet adds that zircon’s X-ray powder pattern is nearly indistinguishable from hafnon, Hf(SiO4), which is 65.97% hafnium.
- Six IMA rows share zircon’s one-cation silicate pattern. A script over the list finds hafnon (Hf), huttonite and thorite (Th), stetindite-(Ce) (Ce), and two with zircon’s own formula, Zr(SiO4): zircon and reidite, approved 2001, USA / Barbados. A formula alone does not separate those two.
- Metamict zircon reads lower. The sheet: radioactive when metamict, isotropic when metamict, greasy when metamict. GIA sorts zircons into high, intermediate and low types by radiation damage, with specific gravity from 3.90 to 4.73. A 20.00 g piece weighs 15.65 to 15.74 g in water at the sheet’s density, but as little as 14.87 g at GIA’s lowest, which crosses corundum’s 14.97 to 15.12 g. Radioactivity questions go to is my mineral radioactive; this page is not advice.
- Not cubic zirconia, and not diamond. The International Gem Society gives cubic zirconia as ZrO2, cubic and laboratory-made, and says it is not synthetic zircon. The diamond question has its own page, is zircon a diamond.
| Name | Formula and source | IMA status, year, country | Hardness | Density or SG | 20.00 g piece in water | What separates it from zircon |
|---|---|---|---|---|---|---|
| Zircon, crystalline (Handbook) | Zr(SiO4), IMA; ZrSiO4, sheet | G, 1789, Sri Lanka | 7.5 | 4.6 to 4.7 | 15.65 to 15.74 g | The reference point; uniaxial (+), 1.925 to 2.015 |
| Zircon, high to low type (GIA) | Same species, radiation-damaged to varying degrees | - | 6 to 7.5 | 3.90 to 4.73 | 14.87 to 15.77 g | Low type: indices 1.810 to 1.815, birefringence down to 0.000 (GIA) |
| Reidite | Zr(SiO4), IMA | A, 2001-013, USA / Barbados | No sheet used | No sheet used | - | Same formula as zircon: structure, not chemistry |
| Hafnon | Hf(SiO4), IMA | A, 1974-018, Mozambique | No sheet used | No sheet used | - | Hafnium for zirconium: 65.97% Hf worked; X-ray pattern nearly indistinguishable (zircon sheet) |
| Baddeleyite | ZrO2, IMA and sheet | G, 1893, Sri Lanka | 6.5 | 5.40 to 6.02 | 16.30 to 16.68 g | Natural zirconium oxide; monoclinic, biaxial (−) |
| Ziroite | ZrO2, IMA | A, 2022-013, Israel | No sheet used | No sheet used | - | The second natural ZrO2 row; no Handbook sheet |
| Cubic zirconia (IGS) | ZrO2, cubic; laboratory-made | No row | 8.5 | 5.34 to 6.00 | 16.25 to 16.67 g | No birefringence (IGS); not a mineral |
| Corundum (Handbook) | Al2O3 | G, 1714 ?, India ? | 9 | 3.98 to 4.10 | 14.97 to 15.12 g | Inside GIA’s zircon range: the line fails here |
What is zircon? Zr(SiO4), as the IMA list and the sheet give it
The names. The September 2026 IMA-CNMNC Master List lists zircon as Zr(SiO4), status G, grandfathered, with the year 1789 and Sri Lanka as the country of first discovery. The Handbook sheet writes ZrSiO4, the same composition. Its Name field says the word is thought to be derived from the Persian zar, for gold, and gun, for colour, through the Arabic zarqun.
What it looks like, per the sheet. Tetragonal, point group 4/m 2/m 2/m, and it may be metamict. Most commonly as tabular to prismatic crystals with square cross sections, terminated by {111}, to 30 cm; also irregular grains and massive; geniculated twins on {101}, forms read from the rendered sheet. Colour reddish brown, yellow, green, blue, grey or colourless, with a white streak and a vitreous to adamantine lustre. Cleavage is indistinct on {110} and {111}, fracture conchoidal, tenacity brittle. Optically uniaxial (+), ω 1.925 to 1.961 and ε 1.980 to 2.015, with very strong dispersion; the sheet also records it as thermoluminescent and cathodoluminescent, and says it may fluoresce under UV.
Where it forms. The Occurrence field: an accessory mineral in igneous and metamorphic rocks, reaching large size in mafic pegmatites and carbonatites, and in sedimentary rocks and alluvial heavy-mineral sands. The Association line reads feldspars, amphiboles, mica, quartz. GIA calls zircon a geological clock for its radioactive trace elements and reports zircon from Australia 4.4 billion years old; this page makes no dating claim of its own.
Is zircon radioactive? What the sheet and GIA record
The sheet’s words. Radioactive when metamict; isotropic when metamict; greasy when metamict. Those three phrases are all the Handbook says. Its one analysis, from Sitarampuram, India, carries 0.027% U3O8 and 0.01% ThO2: worked on standard atomic weights, about 229 parts per million uranium and 88 parts per million thorium in that sample. One analysis is not a range for the species, and this page gives none.
GIA’s account of the damage. GIA says some zircons, usually green ones, show much lower optical and physical values because radioactive elements present as impurities broke down the crystal structure over geological time. It sorts zircons into high, intermediate and low types: high zircons have full structures; low zircons can be practically amorphous. Its figures run from indices of 1.925 to 1.984 for high type to 1.810 to 1.815 for low, birefringence 0.000 to 0.059, hardness 6 to 7.5 and specific gravity 3.90 to 4.73. GIA adds that virtually all zircons used in jewellery are high type, and that high-temperature heat treatment repairs the damaged structure.
What to do with that. Nothing here is health or handling advice. Which species matter for radioactivity, and how a collection is checked, is on is my mineral radioactive; storage is on how to store radioactive mineral specimens.
How heavy is zircon? The 15.65 g line, and where it fails
The rule. Weigh a clean 20.00 g piece in air, then suspended in water. At the sheet’s measured density of 4.6 to 4.7, crystalline zircon reads 15.65 to 15.74 g; at the calculated 4.714, 15.76 g. Quartz reads 12.45 g and topaz 14.27 to 14.40 g, as on the live quartz and topaz pages, so a crystalline zircon sits well clear of both. The method and its error sources are on how to measure specific gravity at home.
Documented failure: metamict zircon. GIA’s specific gravity runs down to 3.90 for low-type zircon, which reads 14.87 g. That crosses corundum, 3.98 to 4.10 on its sheet, 14.97 to 15.12 g. So a reading between 14.87 and 15.65 g says possibly a damaged zircon, possibly something else, and the scale stops deciding. Above the line, zirconium oxide is the next step up: baddeleyite, 5.40 to 6.02 on its sheet, reads 16.30 to 16.68 g, and cubic zirconia, 5.34 to 6.00 on the gem society’s chart, 16.25 to 16.67 g.
The hafnium check, worked. Sitarampuram: SiO2 32.86, ZrO2 66.72, HfO2 1.15. Dividing each oxide by its formula mass and scaling to one silicon gives zirconium 0.990 and hafnium 0.010, which is the (Zr0.99Hf0.01)Si1.00O4 the sheet prints: the method’s positive control. Hafnium is 0.98% of that sample by mass. The scale does not see it; the analysis does.
Zircon or cubic zirconia? Two zirconium compounds
Different substances. The International Gem Society zircon article says zircon and cubic zirconia share a similar name but are completely different: zircon is ZrSiO4 and tetragonal; cubic zirconia is ZrO2, isometric or cubic. It says cubic zirconia used in jewellery is laboratory-made and is not synthetic zircon, and that synthetic zircon has been grown by the flux method for research, with no known jewellery use. GIA calls zircon a gem unfairly confused with cubic zirconia.
Natural ZrO2 exists, under other names. A script over the IMA list finds two rows with the formula ZrO2: baddeleyite, grandfathered, 1893, Sri Lanka, and ziroite, approved 2022, Israel. The Handbook baddeleyite sheet gives it as monoclinic, hardness 6.5. No row is named cubic zirconia or zirconia. The gem-trade material is a made product, not a mineral species; the site’s test of that distinction is on is quartz a mineral.
Colour, per GIA. GIA lists zircon in blue, red, yellow, orange, brown and green, and says zircon’s blue is almost always the result of heat treatment. Colourless zircon, it says, is called Matara zircon after a city in Sri Lanka; the Handbook’s Distribution field spells the district Matura.
Where zircon is found: Mud Tank, Miass, Matura and Renfrew County
The Distribution field opens an unusually common and widely distributed mineral, but fine crystals are rare, and names: Larvik and Hakedal, and Seiland Island, Alta River, Finnmark, Norway; Sillinjärvi, Finland; Sibnitz, Saxony, Germany; Miass, Ilmen Mountains, Southern Urals, Russia; Turner’s Island in Lake Clear, and Kuehl Lake, Renfrew Co., Ontario; the Matura and Saffragam districts, Sri Lanka; Mt. Ampanobe near Fianarantsoa, and Itrongahy near Betroka, Madagascar; Teete, Mozambique; and large crystals from Mud Tank, Valley Bore, Northern Territory, Australia.
In the USA it names Deer Hill, Orange Co., and Fine, St. Lawrence Co., New York; near Franklin, Sussex Co., New Jersey; near Green River, North Carolina; Tigerville, South Carolina; and Cheyenne Mountain and St. Peters Dome near Pikes Peak, Colorado. The sheet names no British locality, and we add none.
Relatives on this site. The diamond sheet lists zircon among the minerals of diamond placers, beside rutile, topaz and garnet; topaz and corundum have their own pages. The rest of the reference section is at the species index. We hold no catalogue and nothing here is offered for sale; the wanted list is the only commercial route.