Cubic Zirconia

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Introduction

This manufactured gemstone, more often referred to just as "C.Z.", first appeared on the market in 1978, and like all new things the price was quite high initially. It was marketed under the name of "Djevalite". Advances in the technology of laboratory producing has brought the price right down.

The first C.Z.'s were made in the USSR - as it was then. The Russians did not have enough natural rubies, whish at that time essential was in the manufacture of lasers. Therefore, C.Z. manufacture was a purely commercial enterprise in an attempt to cheapen laser technology without the need to import rubies. Clearly, the parameters for producing a stone would have been that it must have similar optical properties to a ruby, but did not require the mineralogical properties of corundum. It was not an attempt to produce a diamond substitute. In fact the USSR had found very viable deposits of diamonds by this time.

There was also a "whisper" that it was developed as window glass for the space programme in Russia - a fact that seems somewhat difficult to believe and no reference to this has been found.

Physical Properties

Chemical Composition Zirconium dioxide plus yttrium or calcium
Chemical Formula ZrO2 + Ca or Yt as a stabiliser
Colour Colourless when pure, but small amounts of additives, included when manufacturing, can change it to any desire shade (see notes below).
Hardness 8-8.5 Brittle
Specific Gravity
(Relative Density)
5.5 - 6.0
Crystal Form Cubic (Isometric)
Refractive Index 2.15 - 2.18
Dispersion 0.058 - 0.066

The Manufacturing Process

Zirconium is a metal, which will crystallise in a natural form with silicon, to form the gemstone zircon: a zirconium silicate. However, in nature, this forms in the tetragonal system, and is known as baddeleyite.

In the manufactured process the silicon is substituted for oxygen, and forced to combine unnaturally into the cubic system. This "forcing" is why the tiny amounts of calcium or yttrium are added; to stabilise the zirconium metal to combine and crystallise with the oxygen; an otherwise unnatural process.

It actually takes such a high temperature to melt the zirconium oxide (2,700°C), that it was not possible until the invention of something most of us possess today, the microwave oven, which was invented in the early 1970's.

The incredibly high temperatures required for melting prevents any sort of conventional crucible (a vessel made of a substance with a high melting temperature used for melting materials at high temperatures) being used, even platinum. So the material has to form its own crucible. This process is known as a skull melt. It relies on the fact that it heats up from the inside outwards, so the outside stays relatively cool while the inside melts, thus it forms its own outer crust or "skull".

The powder mixture of zirconium oxide with the stabiliser of calcium or yttrium is placed in the microwave, the heat is turned up and the interior melts but the outer regions remain solid. The heat is switched off and the melt is allowed to cool slowly under strictly controlled conditions and annealed. As this happens the cubic zirconia crystal forms in the cubic system. This is then cut and faceted to produce the finished gemstone.

Diamonds also crystallise in the cubic system, and this is a crucial factor. Any other crystal system would make this stone look "not quite right", even to the layman.

As previously stated, these C.Z.'s can be produced in any colour (this is started at the powder mixing stage of course), and a wide variety of chemicals are employed to achieve this, but for the most part there is more profit in diamond simulation and it is left "pure".

Colouring

A number of various oxides are used to colour. These are added at the powder mix stage. For ease of reference these are laid out in the table below. Vivid green C.Z. is sometimes referred to as C-OX, and C.Z. in numerous colours are often sold under various trade names.

Chemical additive (in addition to small amounts Ca or Yt) Colour produced
None Clear
Cerium oxides - CeO2, Ce2O3 Red orange and yellow
Copper - CuO, Iron - Fe2O3, Nickel - NiO, Praesiodymium - Pr2O3, and Titanium - TiO2 Yellow, amber and brown
Erbium - Er2O3, Europium - Eu2O3, Holmium - Ho2O3 Pink
Chromium - Cr2O3, Thulium - Tm2O3, Vanadium - V2O3 Olive green
Cobalt - Co2O3, Manganese - MnO2, Neodymium - Nd2O3 Lilac and violet
Larger quantities of stabiliser - Calcium - Ca, Yttrium - Yt Sapphire blue, emerald

Cubic Zirconia as a Gemstone Substitute

The trade in synthetic gemstones is comparatively recent. Laboratory produced rubies (boules) were only introduced in about 1900. It was primarily the development of synthetic rubies for industry - watch pivots - rather than the jewellery trade that started off boule production. But like all these things, someone thought that there might be a market for them. This was not the case with synthetic diamonds, which started to be produced towards the end of the 20th Century and which was purely commercial. The Russians can now produce laboratory grown diamonds up to 5 cts, and an American company will turn your cremated ashes into a diamond with your name laser etched into it - naturally this costs more than the diamond is worth!

With the advent of high temperature microwaves, it suddenly became possible to produce a very realistic diamond substitute. Prior to this there was only "paste", quartz or glass, which would not stand up to close scrutiny or wear. There is a belief among some jewellers that because C.Z. are relatively cheap setting them in gold of 14 cts or less is acceptable. Others feel that C.Z. is a "stone" in its own right and appreciate the beauty of it for its own sake. So to some extent the jewellery trade has "degraded" this stone, but they still sell well because of the considerably lower price. We would all like "the real thing", but often finances dictate otherwise.

How to Identify Cubic Zirconia

To be honest, it is virtually impossible to the layman to identify C.Z correctly. It takes an expert or instruments to tell C.Z. and Diamond apart. But when place side by side, it seems that the cubic zirconia has the higher lustre. This is because it is synthetic, and unlike a natural stone has no inclusions. Lack of these inclusions is extremely rare in natural stones. Most jewellers have a refractometer, and every gemstone has its own refractive index - the R.I. - in simple terms the angle at which light "bounces" off. It is as necessary for a jeweller to have one of these as a hand lens or a streak plate is necessary to a geologist.

If we take as an instance the gemstone corundum, many people are aware this comprises rubies and sapphires and the amount of aluminium which dictates whether it is a ruby or sapphire. However rubies vary in hue, although always a "shade of red" (from orangy to brownish depending where they come from), this is not the case with sapphires which are normally blue, but can also be red, green, clear, and even purple enough to be thought an amethyst. Similarly topaz comes in a wide variety of colours: yellow, brown, blueish, greenish. Although the name beryl is correct and is a gemstone in its own right, beryl is also a family name encompassing aquamarine (blue green), emerald (green), golden beryl (golden yellow to straw), morganite (rose to rose pink), heliodor (yellow with green to honey tint) and goshenite (colourless to white). These are just a few examples to show that gemmology is a discipline in its own right.

Clear C.Z. look just like a good diamond, but they are considerably heavier (65% heavier) and have a different R.I. That brings us to another problem. The R.I. of C.Z. is outside the range of a normal refractometer and a specialist version called a Jemeter has to be used. Also they are "softer" on the Mohs Scale of Hardness than a diamond; although most jewellers might take umbrage if you got your test kit out! When coloured, the only real test is R.I. (with a Jemeter), X ray or a mass spectrometer. Since the latter two mean that the stone must be powdered they are not usually practical tests.

An experienced jeweller will look for "rounding" on the facet junctions. C.Z. will not have that very sharp junction that a diamond will have, but it does take an experienced eye to see it. Similarly C.Z. allows light to pass through at angles a diamond will not. This is known in the trade as "reading". A further test is thermal conductivity, but again a specialist meter is required. However, in both these instances the man-made gemstone moissanite gives false readings

Diamonds

The hardest naturally occurring substance on Earth, it has been valued by man since ancient times. There really is so much to say about diamonds it could form a separate article. Therefore this article will confine itself to a simple reminder of the physical properties table, purely for comparison purposes.

Chemical name & formula Diamond. C (polymorphous with chaoite, graphite & lonsdaleite)
Chemical composition Pure carbon
Hardness 10 brittle
Streak White
Colour Colourless, grey, blueish, greenish, yellowish, brown and black.
Transparency Transparent to opaque
Lustre Adamantine
Cleavage Perfect on {111}
Fracture Concoidal
Specific gravity (Relative density) 3.52
Morphology Crystals, twinning. Twinning on {111} and {001} common
Crystal system Cubic
Crystal form Octahedra, dodecahedra, cubes
Luminescence Blue to greenish
Radio activity None usually
Admixes Si, Al, Mg, Fe
Chemical properties Insoluble both in acids and in alkalis
Handling Clean with water or dilute acids
Similar minerals none
Distinguishing features Hardness
Genesis Magmatic, in alluvial deposits and meteorites
Associated minerals Gold, platinum, magnetite, rutile, olivine, garnet var. pyrope, & zircon
Occurrence Rare: in ultrabasic rocks (kimberlite) and alluvial deposits, occasionally in meteorites. The most important deposits are in RSA in the vicinity of Kimberley (in the alluvial deposits of the Vaal and Orange Rivers; in the premier mine the largest diamond crystal found weighed 622 gms (3,106 carats - known as The Cullian). Also in the eastern part of the Deccan Plateau, ( India), (Namibia), (Angola), Kasai Province (Ziare), Sierra Leone, (Ghana), Borneo), Brazil), western parts of the Urals, Yakutia (Russia), (Australia), Arizona (USA)
Application Precious stone; for cutting, grinding and drilling tools

Taken (in the main) from Duda and Rejl (1998)

Zircon

This naturally occurring mineral should not be confused with cubic zirconia, although many people tend to believe they are the same.

Again this is a gemstone which occurs in many forms. Colours include yellow (jargon), orange, red, (hyacinth) brown, green and blue (star light), with this striking blue being the most attractive and the most sought after. It rarely occurs clear.

The colour variation comes from the radio active decay of tiny amounts of uranium and thorium, which disturbs the crystal lattice, breaking it down into silica and zirconium. A heat process to produce reverses this decay of the structure.

Physical properties:-

Chemical name & formula Zirconium Silicate: ZrSiO4
Chemical composition ZrO2 67.01%, SiO2 32.99%
Hardness 7.5 brittle
Streak White
Colour Colourless. yellow (jargon variety), reddish to orange red (hyacith variety) greenish and blueish (starlight variety), brown
Transparency Transparent to translucent to opaque.
Lustre Vitreous, adamantine, greasy.
Cleavage Imperfect on {100} and {110} poor {111}
Fracture Uneven to concoidal
Specific gravity (Relative density) 4.0 to 4.7
Morphology Crystals, granular, earthy and radiating aggregates, grains and pebbles in alluvial deposits
Crystal system tetrogonal
Crystal form Prismatic, dipyramidal, isometric. Twinning on {111}, {100} & {221} rare
Luminescence Yellow, orange, dull red, green-yellow.
Radio activity Slightly radioactive whenit contains admixures U and Th
Admixes U, Th, rare earths, H2O(malacon, cyrtolite varieties), Y, Nb (neagite variety), Hf (alvite variety), rare earths, (oyamalite variety)
Chemical properties Infusible, partly soluble in H2SO4, HF & HCl. Zircons are often found in the matamict state
Handling Clean with water
Similar minerals Xenotime, thorite, garnet
Distinguishing features Hardness, specific gravity, x-rays, chemical properties
Genesis Magmatic, metamorphic, pegmatic, sedimentary, in alluvial deposits.
Associated minerals Biotite, amphibole, quartz, garnet
Occurrence Common. Laacher See, Niedermendig (Germany), Arendal, Langensunjanfiord (Norway), Ilman Mnts [crystals up to 3.5 kg], Miass, Ural Mnts., Slijudanka [crystals up to 5 cm] (Russia), Expailly, St Michael, Espaly-Saint-Marcel (France), Haddem, Connecticut, Mellam, Wisconsin [needles up to 20 cms] (USA) Renfrew Co. [crystals up to 7 kg] (Canada), (Madagascar), Mogok region (Burma) Palin (Cambodia), )Sri Lanka) New South Wales (Australia), Minas Grande (Brazil).
Application Zr source, also used as a gemstone. (facets, cabochons)
Refractive index 1.92 to 1.98
Bi- refringence 0.059 uniaxial and positive

Taken (in the main) from Duda and Rejl (1998)

Conclusion

The main makers of C.Z. do make a full disclosure that their stones are laboratory manufactured. This however does not prevent unscrupulous jewellers being less forthcoming when selling on to the public. Large expensive diamonds come with a provenance but of course the sort most of us are able to afford do not.

Whilst we have the Trades Description Act and Office of Fair Trading to call on, we could only do so if we suspected fraud. Since it is impossible for the layman to tell the difference with the naked eye it must finally come down to what each individual can afford, or the wish to own "the real thing".

Some people consider that a C.Z. is purer than a natural diamond, since the only inclusions will be those which are meant to be there, and the structure is always perfect.

Therefore, if that is to be the deciding factor go for C.Z. It will be cheaper and very few people will ever know the difference.

References.

Rudolf Duda and Lubos Rejl, 1998. Minerals of the World. Spring Books (Hamlyn)

Rudolf Duda and Lubos Rejl, 2002. World of Gemstones. Geoscience Press

Fleisher and Mandarino, 1995. Glossary of Mineral Species Seventh Edition. The Mineralogical Record Inc.

Rutley's Elements of Minerology, 27th Edition, revised by C.D. Gribble. UnwinHymen.

Roberts, Campbell and Rapp. Encyclopedia of Minerals, second edition. Van Nostrand Reinhold.

http://www.allsands.com/Fashion/cubiczirconia_xfv_gn.html

http://www.emporia.edu/earthsci/amber/go340/students/berg/c.html

http://www.theimage.com/gemstone/synthetic/synthetic.html

http://www.tradeshop.com/gems/cz.html

N.B. There are vast numbers of hits on the web for "cubic zirconium" - the above are merely a sample.