Synthetic sapphiresBy:Ardamun Ramdeva
Summary: Having identical chemical composition and physical properties of natural sapphires, synthetic sapphires, over the years, have achieved a high degree of perfection. The many different methods of producing synthetic sapphires are: melt growth, solution growth, or extremely high-temperature, high-pressure growth processes.
Synthetic Sapphires - Gemstones Par Excellence
Sapphire was first synthesized in 1902. Synthetic Ruby - the 'red sapphire',
though, was produced in 1888 by the French chemist, Auguste Victor Louis
Verneuil, through flame fusion process. A synthetic sapphire or the one
created in a lab, under controlled conditions, has crystal structure,
optical properties and chemical composition of its natural avatar. Some
synthetic processes simulate the geological conditions required to produce
natural sapphire. Both, Sapphire and Ruby, are varieties of corundum, a
crystalline form of aluminum oxide. In nature, corundum comes in various
shades of blue, as well as colorless, red, yellow, pink, orange, brown,
purple, and green. Corundum of all colors is known as Sapphire, except the
red sapphire, which is known as Ruby.
The word sapphire is known to originate from the Latin 'sapphärus', Middle
English 'saphir', Old French 'safir', Greek 'sappheiros', and Hebrew 'sappîr'.
Production of Synthetic Sapphires
Synthetic sapphires come in almost all colors, but the most sought after is
the deep blue sapphire. Basically, to get different colors, dopants are
added to aluminum oxide. Sapphires are synthesized by melting these at over
2000º Celsius. Having identical chemical composition and physical properties
of natural sapphires, synthetic sapphires, over the years, have achieved a
high degree of perfection. The many different methods of producing synthetic
sapphires are: melt growth, solution growth, or extremely high-temperature,
high-pressure growth processes.
The flame fusion process of Auguste Verneuil involves producing a boule from
finely ground form of aluminum oxide, or Alumna, with the help of an
inverted oxyhydrogen torch. Boule is a mass of alumna with the same physical
and chemical properties of corundum. Oxygen passes through a chamber
containing highly purified alumna, and this is carried to the central part
of the oxyhydrogen flame. The fine alumna particles fuse and fall down as
molten boule. Flame, rate of the feed of alumna, and the lowering of boule
are controlled and adjusted to produce a boule of uniform size and shape. To
produce colorless sapphire, the melting point of boule is 2030º Celsius. The
furnace is shut down once the boule reaches the desired size of 150 or 200
carats, and is allowed to cool.
The Kyropoulos method of producing synthetic sapphires involves
growing sapphire crystals under controlled conditions. During the process of
crystallization, sapphire crystals sprout in cylindrical shapes. The
required diameter of the crystal is maintained by the automatic displacement
of the seed crystal, without rotation. The melt is achieved through
resistive heating. The Kyropoulos method produces sapphire with minimal
mechanical stress.
The Czochralski (pulling) process involves melting the powdered
ingredients in a platinum, iridium, graphite, or ceramic crucible. A
rotating rod, with a seed crystal attached to one end, is lowered to touch
the melt. The rod is slowly withdrawn at the rate of 1 to 100 millimeters
(mm) per hour. As the seed pulls the material from the melt, it cools and
solidifies. Very high purity large crystals of more than 50 mm in diameter
and 1 meter in length are grown by this method.
Some of the other processes for producing synthetic sapphires are: the
floating zone process, the flux process, and the hydrothermal
process.
Uses of Synthetic Sapphires
There are many uses of synthetic sapphires - as gemstone, as well as
non-gemstone. Sharing the physical, chemical and optical qualities and
properties of the natural sapphire, the synthetic sapphires are used
extensively in jewelry. Some of the synthetic sapphires are so perfect that
it becomes difficult to identify them, unless by an experienced jeweler or a
gemologist. Padparadscha sapphires are rare orange-pink variety of sapphires
and have a higher value than even the blue sapphires. These sapphires have
been created in labs. They are so perfect that a few years ago, the
synthetic Padparadscha sapphires were rated as natural till the American Gem
Trade Association (AGTA) found out they were not so.
Jewelry produced by synthetic sapphires is just as beautiful as the one by
natural sapphires. Though not inexpensive, synthetic sapphire jewelry costs
less than that of natural sapphire. Synthetic sapphires range in price and
size, and the smaller sapphires are used in less expensive jewelry. As a
matter of ethics, the origin of the sapphire should always be disclosed.
Though synthetic sapphires can technically be called 'genuine', they are
certainly not natural. One should learn about synthetic sapphires before
buying jewelry, to know what one is buying.
Other applications of synthetic sapphires include their use in watch and
semi-conductor industries. It was first used as watch crystals in the 1960s.
It is used in most high-end watch brands. Synthetic sapphire crystals are
grown in cylindrical ingots. These ingots of pure synthetic sapphires are
sliced into wafers. These are polished into transparent crystal slices. High
quality watches use these slices as watch faces. The exceptional hardness of
the material makes the watch face impossible to scratch.
Wafers of single crystal synthetic sapphires are used in semi-conductor
industry for light emitting diodes. For its extreme abrasive resistance,
synthetic sapphire is the jewel of choice. Its other features include, zero
porosity, extreme hardness, low-friction coefficient, chemical inertness -
even to hydrofluoric acid, high thermal conductivity, and excellent wear
resistance. Due to zero porosity, micro-finishes of two can be attained.
Precision jeweled bearing rollers use sapphire balls for their low-friction
and long-wear applications. Some advantages of using synthetic sapphires as
jeweled bearings are:
* Low friction, hardness and shock resistance
* Close tolerances
* Accurate wear life
* Non magnetic properties
* Resistance to heat, corrosion, and distortion
Due to their high melting point of 2000º Celsius, they are suitable for
high-heat applications also. The other uses of synthetic sapphires include
electronics, optoelectronics, optics, and laser technologies.
Identification of Synthetic Sapphires
* Presence of round, elliptical or flask shaped bubbles.
* Whitish sheen seen in natural sapphires is missing in synthetics
* Strain cracks, curved striae (fine thread-line lines or streaks), wispy
white veils, etc
* Flux inclusions and non-natural gas inclusions
* Visible seed crystals in synthetic sapphires
* Color zones seen when dipped in Methylene Iodide.
* Absorption spectra not seen in synthetic sapphires.
* Unnatural looking colors.
* Dark un-fused blotches seen. Caused by coloring matter.
* Variation in properties of fluorescence.
* Transparency to Ultra Violet light varies.
* Star effect in synthetic sapphires seems to be painted.
* The synthetics have a neatly ground out base as against the lumpy base of
natural sapphires.
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