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FACTS ABOUT CUBIC ZIRCONIA
Cubic zirconia (or CZ), is the
cubic crystalline form of zirconium dioxide (ZrO2). The synthesized material is hard,
optically flawless and usually colorless, but may be made in a variety of different
colors. It should not be confused with zircon, which is a zirconium silicate (ZrSiO4).
Because of its low cost, durability, and close visual likeness to diamond, synthetic
cubic zirconia has remained the most gemologically and economically important competitor
for diamonds since 1976. Its main competition as a synthetic gemstone is the more
recently cultivated material, synthetic moissanite.
As its name would
imply, cubic zirconia is crystallographically isometric and, as diamond is also isometric,
this is an important attribute of a would-be diamond simulant. During synthesis zirconium
oxide would otherwise form monoclinic crystals, its stable form under normal atmospheric
conditions. The stabilizer is required for cubic crystal formation; it may be typically
either yttrium or calcium oxide, the amount and stabilizer used depending on the
many recipes of individual manufacturers. Therefore the physical and optical properties
of synthesized CZ vary, all values being ranges.
It is a dense substance, with a specific gravity between 5.6—6.0 – at least 1.6 times
as dense as diamond. Cubic zirconia is relatively hard, at about 8 on the Mohs scale–
much harder than most natural gems. Its refractive index is high at 2.15—2.18
(B-G interval, compared to 2.42 for diamonds) and its luster is subadamantine. Its
dispersion is very high at 0.058—0.066, exceeding that of diamond (0.044). Cubic
zirconia has no cleavage and exhibits a conchoidal fracture. Because of its high
hardness, it is generally considered brittle.
Under shortwave UV cubic zirconia typically luminesces a yellow, greenish yellow
or "beige". Under longwave UV the effect is greatly diminished, with a
whitish glow sometimes being seen. Colored stones may show a strong, complex rare
earth absorption spectrum.
Discovered in 1892,
the yellowish monoclinic mineral baddeleyite is a natural form of zirconium oxide.
It has little economic importance because of its rarity.
The extremely high melting point of zirconia (2750°C) posed a hurdle to controlled
single-crystal growth, as no existing crucible could hold it in its molten state.
However, stabilization of zirconium oxide had been realized early on, with the synthetic
product stabilized zirconia introduced in 1930. Although cubic, it was in the form
of a polycrystalline ceramic: it was made use of as a refractory material, highly
resistant to chemical and thermal (up to 2540°C) attack.
Seven years later, German mineralogists M. V. Stackelberg and K. Chudoba discovered
naturally occurring cubic zirconia in the form of microscopic grains included in
metamict zircon. Thought to be a byproduct of the metamictization process, the two
scientists did not think the mineral important enough to formally name. The discovery
was confirmed through x-ray diffraction, proving the existence of a natural counterpart
to the synthetic product.
As with the majority of grown diamond look-alikes, the conceptual birth of single-crystal
cubic zirconia began in the minds of scientists seeking a new and versatile material
for use in lasers and other optical applications. Its evolution would eclipse earlier
synthetics, such as synthetic strontium titanate, synthetic rutile, YAG (Yttrium
Aluminium Garnet) and GGG (Gadolinium Gallium Garnet).
Some of the earliest research into controlled single-crystal growth of cubic zirconia
occurred in 1960s France, much work being done by Y. Roulin and R. Collongues. This
technique involved molten zirconia being contained within a thin shell of still-solid
zirconia, with crystal growth from the melt: The process was named cold crucible,
an allusion to the system of water cooling used. Though promising, these pursuits
yielded only small crystals.
Later, Soviet scientists under V. V. Osiko at the Lebedev Physical Institute in Moscow
perfected the technique, which was then named skull crucible (an allusion either
to the shape of the water-cooled container or to the occasional form of crystals
grown). They named the jewel Fianit, but the name was not used outside of the USSR.
Their breakthrough was published in 1973, and commercial production began in 1976.
By 1980 annual global production had reached 50 million carats (10 tonnes).
The Soviet-perfected skull crucible
is still used today, with little variation. Water-filled copper pipes provide a cup-shaped
scaffold in which the zirconia feed powder is packed, the whole contraption being
wrapped with radio frequency induction coils running perpendicular to the copper
pipes. A stabilizer is mixed with the feed powder, being typically calcium oxide.
The RF induction coils function in a manner similar to the primary winding in a transformer.
The zirconia acts as the "secondary winding" of a transformer which in
effect is "shorted" out and thus gets hot. This heating method requires
the introduction of small pieces of zirconium metal. The metal is placed near the
outside of the charge and is melted by the RF coils and heats the surrounding zirconia
powder from the outside inwards. The cooling water-filled pipes embracing the outer
surface maintain a thin "skin" (1-2 mm) of unmelted feed, creating a self-contained
apparatus. After several hours the heat is reduced in a controlled and gradual manner,
resulting in the formation of flawless columnar crystals. Prolonged annealing at
1400°C is then carried out to remove any strain. The annealed crystals, which
are typically 5 cm long by 2.5 cm wide (although they may be grown much larger),
are then cut into gemstones.
The addition of certain metal oxide dopants into the feed powder results in a variety
of vibrant colors. For example:
n recent years manufacturers have sought ways of distinguishing their
product by supposedly "improving" cubic zirconia. Coating finished CZs
in a film of diamond-like carbon (DLC) or Amorphous Diamond is one such innovation,
a process using chemical vapor deposition. The resulting material is purportedly
harder, more lustrous and more like diamond overall: The coating is thought to quench
the excess fire of CZ, while improving its refractive index, thus bringing it more
in line with diamond. Additionally, because of the high percentage of diamond bonds
in the amorphous diamond coating, the finished simulant will show a positive diamond
signature under Raman spectroscopy.
Another technique first applied to quartz and topaz has also been adapted to cubic
zirconia: Vacuum-sputtering an extremely thin layer of metal oxide (typically gold)
onto the finished stones creates an iridescent effect. This material is marketed
as "mystic" by many dealers. Unlike DLC, the surreal effect is not permanent,
as abrasion easily removes the oxide layer.
There are a few key
features of cubic zirconia which distinguish it from diamond, some observable only
under the microscope or loupe, for example:
* Dispersion: With a dispersive power greater than diamond (0.060 vs. 0.044) the
more prismatic fire of cubic zirconia can be seen by even an untrained eye.[citation
* Hardness: Cubic zirconia has a rating of approximately 8 on the Mohs hardness scale
vs. a rating of 10 for diamonds.
* Specific gravity: Cubic zirconia crystals are heavyweights in comparison to diamonds;
a cubic zirconia will weigh about 1.7 times more than a diamond of equivalent size.
* Flaws: Contemporary production of cubic zirconia is virtually flawless,[citation
needed] whereas most diamonds have some sort of defect, be it a feather, included
crystal, or perhaps a remnant of an original crystal face (e.g. trigons).
* Refractive index: Cubic zirconia has a refractive index of 2.176, compared to a
* Cut: Some cubic zirconias use different facet shapes than are typically used for
diamonds. This difference would be visible under close inspection with a loupe.
* Color (or more precisely, the lack thereof): Only the rarest of diamonds are truly
colorless, most having a tinge of yellow or brown to some extent. By comparison,
cubic zirconia can be made in most cases entirely colorless: equivalent to a perfect
"D" on diamond's color grading scale.
* Thermal conductivity: Cubic zirconias are thermal insulators while diamonds are
among the most efficient thermal conductors, exceeding copper. This makes telling
the difference between diamond and cubic zirconia quite easy for those with the right
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