Is zircon a diamond?
No. Zircon is Zr(SiO4), hardness 7.5; diamond is carbon, hardness 10. Of 12 diamond names on a gem society list of false names, none is diamond; two are zircon.
No. Zircon is a zirconium silicate, Zr(SiO4): hardness 7.5, density 4.6 to 4.7, doubly refractive. Diamond is carbon: hardness 10, density 3.511, singly refractive. Cubic zirconia is a third thing, laboratory-made ZrO2. Of the 12 diamond names on a gem society list of false names, none is diamond and two are zircon.
In short
- Three different materials. The IMA-CNMNC Master List lists zircon, Zr(SiO4), grandfathered, 1789, Sri Lanka, and diamond, C, grandfathered, year and country unknown. A script over the list finds no row called cubic zirconia; the International Gem Society describes it as ZrO2, cubic and laboratory-made. The two natural ZrO2 minerals on the list are baddeleyite and ziroite.
- The 12-name count. The gem society’s list of false or misleading gem names, taken from Schumann’s Gemstones of the World, has 57 entries. Twelve contain the word diamond. By script, six of those are rock crystal quartz, two zircon (Ceylon diamond and Matura diamond), two colourless topaz and two glass imitation. None is diamond.
- The doubling rule. The gem society says diamonds are singly refractive and that zircon’s strong birefringence doubles the back facets seen through the crown. Crisp single facets fit diamond, cubic zirconia, spinel, glass, GGG or YAG; doubling fits moissanite, zircon, topaz, quartz or sapphire. On the Handbook sheets zircon’s two indices are about 0.055 apart; diamond’s is one number.
- Documented failure: damaged zircon. GIA gives zircon a birefringence of 0.000 to 0.059, from low to high type. A low, radiation-damaged zircon can show no doubling at all, so a single image proves nothing on its own. GIA says virtually all jewellery zircons are high type.
- Weight for size, worked. A stone the size of a 1.00 ct diamond weighs 1.31 to 1.34 ct as zircon and 1.52 to 1.71 ct as cubic zirconia, but 0.99 to 1.02 ct as topaz. The gem society says topaz, spinel and moissanite have densities similar to diamond’s, and the arithmetic agrees: size and weight cannot clear topaz.
| Material | What it is | Hardness | Density or SG | Same size as 1.00 ct diamond | Refraction | Back facets (IGS) | Its names on the IGS false-name list |
|---|---|---|---|---|---|---|---|
| Diamond (Handbook) | Species, C | 10 | 3.511 | 1.00 ct | Single: n = 2.4175, isotropic | Single, crisp | None of the 12 “diamond” names |
| Zircon, high type (Handbook) | Species, Zr(SiO4) | 7.5 | 4.6 to 4.7 | 1.31 to 1.34 ct | Double: 1.925 to 1.961 and 1.980 to 2.015 | Doubled, plainly | Ceylon diamond; Matura diamond |
| Zircon, low type (GIA) | Same species, radiation-damaged | 6 | 3.90 | 1.11 ct | 1.810 to 1.815; birefringence to 0.000 | May show none: the rule fails | - |
| Cubic zirconia (IGS) | Laboratory-made ZrO2; no IMA row | 8.5 | 5.34 to 6.00 | 1.52 to 1.71 ct | Single; no birefringence | Single, crisp | - |
| Moissanite (IGS chart) | SiC; the natural mineral has an IMA row, G, 1905 | Not used | 3.17 to 3.20 | 0.90 to 0.91 ct | Birefringence 0.043 | Doubled | - |
| Colourless topaz (Handbook) | Species, Al2SiO4F2 | 8 | 3.49 to 3.57 | 0.99 to 1.02 ct | Double: 1.606 to 1.644 | Doubled | Saxon diamond; slave-diamond |
| Rock crystal quartz (Handbook) | Species, SiO2 | 7 | 2.65 | 0.75 ct | Double: 1.544 and 1.553 | Doubled | Alaska, Arkansas, Bohemian, German, Marmarosch, Mexican diamond |
| Glass (IGS list) | Imitation | Not used | Not used | - | Single | Single | Simili diamond; Strass diamond |
Is zircon a diamond? The criteria that decide it
Composition first. Diamond is carbon: the Handbook of Mineralogy diamond sheet says nearly pure carbon, and GIA puts it at typically about 99.95%. Zircon is Zr(SiO4) on the IMA list, 49.77% zirconium by mass, worked. They share no essential element, so no zircon is a diamond and no diamond a zircon; the question is only ever about appearance.
Why the confusion is old. GIA says colourless zircon’s brilliance and fire are close enough to diamond’s to account for centuries of confusion, and that colourless zircon was widely used as a diamond alternative in the nineteenth century. The International Gem Society says colourless zircons have been sold as Matara or Matura diamonds and Ceylon diamonds, and that selling them as actual diamonds is unethical.
The criteria, in the order a collector can apply them: the name on the label, read against the false-name list; the back facets, single or doubled; and the weight for the size. Hardness separates them on paper, 10 against 7.5, but a scratch test is not a way to examine a cut stone; how to test mineral hardness properly explains why. Zircon’s own data, including the metamict types, are on zircon, and the identification guides are at the collecting guides.
The 12 diamond names, and what each one really is
Method. The International Gem Society’s List of False or Misleading Gemstone Names, credited to Walter Schumann’s Gemstones of the World, 3rd edition, was saved and parsed by script into 57 pairs of false name and actual gemmological name. The script kept every false name containing the word diamond.
The count. Twelve names. Six are rock crystal, that is quartz: Alaska, Arkansas, Bohemian, German, Marmarosch and Mexican diamond. Two are zircon: Ceylon diamond, colourless zircon, and Matura diamond, which the list calls colourless fired zircon. Two are colourless topaz: Saxon diamond and slave-diamond. Two are glass imitation: simili diamond and strass diamond. None of the 12 is diamond. A place name in front of the word diamond is, on this list, always a warning.
Matara or Matura. GIA writes Matara, after a city in Sri Lanka near where colourless zircon is mined; the gem society writes both; the Handbook zircon sheet names the Matura and Saffragam districts of Sri Lanka in its Distribution field. The spelling differs; the stone is zircon.
Doubled back facets: the doubling rule, and where it fails
The rule, from the gem society. Look through the crown at the back facets with a loupe, from several angles. Diamond is singly refractive. If the facets are always single and crisp, the stone could be diamond, cubic zirconia, white spinel, glass, GGG or YAG. If they are doubled, it might be moissanite, white zircon, white topaz, quartz or white sapphire, and it is not diamond. The gem society says the effect is often obvious in zircon and subtler in other substitutes.
Why zircon doubles. The zircon sheet gives ω 1.925 to 1.961 and ε 1.980 to 2.015, about 0.055 apart at either end of the range; the diamond sheet gives one index, 2.4175 at 589 nm, isotropic. GIA calls zircon’s pronounced double refraction the reason you see twice as many facets.
Documented failure. GIA’s birefringence for zircon runs from 0.000 for low type to 0.059 for high type, because radiation damage destroys the structure that splits the light. A low zircon can therefore show single facets, the diamond result. Doubling rules diamond out; single facets do not rule it in, which is why the gem society’s first test is a thermal diamond tester, not a loupe.
Zircon or cubic zirconia: which one is the diamond look-alike?
Both, historically. The International Gem Society zircon article says zircon and cubic zirconia have both been used extensively as diamond simulants, and that they are different gems: zircon ZrSiO4, tetragonal; cubic zirconia ZrO2, cubic. Its cubic zirconia article says Russian scientists perfected growing single crystals in the 1970s and gives a hardness of 8.5.
Weight for size, worked. Take the volume of a 1.00 ct diamond at the sheet’s density, 3.511. The same volume weighs 1.31 to 1.34 ct as zircon, 1.52 to 1.71 ct as cubic zirconia on the specific gravity of 5.34 to 6.00 in the gem society’s diamond look-alike chart, and 0.75 ct as quartz. Topaz, 0.99 to 1.02 ct, is the documented failure: it matches diamond within a few points. The home method for density is on how to measure specific gravity at home.
Who each answer is wrong for. “Diamond” is wrong for any stone with doubled back facets. “Zircon” is wrong for a stone with no doubling and a weight near 1.5 ct for a 1 ct diamond’s size, which points to cubic zirconia. “Cubic zirconia” is wrong on a mineral label: it has no IMA row, unlike baddeleyite and ziroite, the natural ZrO2 minerals. Diamond’s carbon relative is on graphite. What we look for is on the wanted list.