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CUBIC ZIRCONIA
Cubic Zirconia (or CZ) is zirconium oxide (ZrO2),
a mineral that is extremely rare in nature but is widely
synthesized for use as a diamond simulant. 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 diamond simulant
since 1976. Its main competition as a synthetic gemstone
is the more recently cultivated material simulated moissanite.
Technical Aspects of Cubic Zirconia
Cubic zirconia is, as its name would imply, crystallographically
isometric, and as diamond is also isometric, this is an
important attribute of a would-be diamond simulant. Synthesized
material contains a certain mole percentage (10-15%) of
metal oxide stabilizer. During synthesis zirconium oxide
would otherwise form monoclinic crystals, as that is 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, syntheic cubic zirconia,vary, all values being ranges.
It is a dense substance, with a specific gravity between
5.6 - 6.0. Cubic zirconia is relatively hard, at about 8.5
on the Mohs scale - nowhere near diamond, but much harder
than most natural gems. Its refractive index is high at
2.15 - 2.18 (B-G interval) 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. It is considered brittle.
Under shortwave UV cubic zirconia typically luminesces a
yellow, greenish yellow or "beige." Under longwave
UV the effect is greatly diminished, with sometimes a whitish
glow being seen. Colored stones may show a strong, complex
rare earth absorption spectrum.
Cubic Zirconia (CZ) History
Since 1892 the yellowish, monoclinic mineral baddeleyite
had been the only natural form of zirconium oxide known.
Being of rare occurrence it had little economic importance.
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 a natural counterpart to the synthetic
product exists.
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 (10,000 kg). See also Russian Star.
Synthesis of Cubic Zirconia
Larry P Kelley monitoring melting zirconium oxide in a furnace
to create cubic zirconia.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 either yttria or calcium oxide.
The RF induction coils function in a manner similar to the
primary winding in a transformer. The heated zirconia acts
as the "secondary winding" of a transformer which
in effect is "shorted" out and thus gets incredibly
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:
Cerium: yellow, orange, red
Chromium: green
Neodymium: purple
Erbium: pink
Titanium: golden brown
Innovations of CZ (Cubic Zirconia)
In 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. See Russian Star.
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.
CZ (Cubic Zirconia) versus Diamond
Cubic zirconia is so optically close to diamond that only
a trained eye can easily differentiate the two. There are
a few key features of CZ 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
CZ can be seen by even an untrained eye.
Hardness. CZ has an 8.5 to 9.0 on the Mohs'
hardness scale vs. a rating of 10 for diamonds.
Specific gravity. CZs are heavyweights in
comparison to diamonds; a CZ will weigh about 1.7 times
more than a diamond of equivalent size. Obviously, this
difference is only useful when examining loose stones.
Flaws. Contemporary production of cubic zirconia
is virtually flawless, 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. CZ has a refractive index
of 2.176, compared to a diamond's 2.417.
Cut. Under close inspection with a loupe,
the facet shapes of some CZs appear different from diamonds.
In
theory, many gems (such as CZs and diamonds) look best when
the star facet, crown main facets, and upper girdle facets
do not quite meet. (Per Step 11 of editor's note 36 to Marcel
Tolkowsky's Diamond Design.) Diamond has such a high refractive
index that having these facets meet at a single point does
not cause much loss of fire or reflection. Diamonds normally
have these facets meet at a point, because that is more
symmetrical and reflects well on the cutter's precision.
On the other hand, CZ has a considerably lower refractive
index than diamond. CZs are often cut with 6-sided crown
main facets, so that the star facets do not touch the upper
girdle facets. This optimizes the brilliance and fire of
the CZs.
The
optimum angle of the main crown facets is steeper for diamond
than for CZ. (According to Tolkowsky's model of the crown,
for a given pavilion angle and girdle thickness). CZs are
often cut so that the crown main facets do not touch the
girdle. This allows the CZs to have a shallower crown angle,
while still having the same crown height as a diamond with
a similar cut.
Color. More precisely, the lack of color:
Only the rarest of diamonds are truly colorless, most having
a tinge of yellow or brown to some extent. By comparison,
CZ can be made in most cases entirely colorless: equivalent
to a perfect "D" on diamond's color grading scale.
Thermal conductivity. CZs are thermal insulators
whilst diamonds are among the most efficient thermal conductors,
exceeding copper. This makes telling the difference between
diamond and CZ quite easy for those with the right tools.
Caring for Cubic Zirconia
Cubic Zirconia can be cleaned with any conventional jewelry
cleaner or detergent. With such a high hardness and
durability, you can use a brush to clean off dirt or oil
also. Ultra clean jewelry cleaners may also be used
on CZ and will not damage the stone. However, when
using soap leaving a film that will dull the brilliance
of the stone. Cubic zirconia should be cleaned frequently
to remove oils from skin that also dull the brilliance of
the gem.
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Source:
Wikipedia contributors. Cubic zirconia [Internet]. Wikipedia,
The Free Encyclopedia; 2007 Jan 1, 04:12 UTC [cited 2007 Jan 6]
Available from:
http://en.wikipedia.org/w/index.php?title=Cubic_zirconia&oldid=97665489.
Tags :
synthetic gemstone,
cubic zirconia Natural Gemstone,
Marcassite Gemstone