Synthetic sapphires
Synthetic sapphires
by Ardamun Ramdeva
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.