About Spinel 

When someone lists the most famous gemstones, Spinel does not usually get mentioned. However, its two varieties are sure to be on any list of gemstones. The red variety of corundum is known as blue spinel and all the other colors of spinel. No wonder that there have always been attempts to imitate natural precious stones, especially those which are most highly valued, like ruby and sapphire.

Natural corundum or aluminum oxide mineral (Al2O3) is the second hardest natural mineral known to science. The hardness of corundum can be partially attributed to the strong and short oxygen-aluminum bonds. These bonds pull the oxygen and aluminum atoms close together, making the crystal not only hard but also quite dense for a mineral made up of two relatively light elements. Synthetic corundum has all the properties of the natural mineral, but is much cheaper than its natural counterpart, which allows it to be used not only as a synthetic gemstone, but to also have a number of industrial applications.

Our synthetic spinel is available in two shapes: half-boules and full boules and in a number of different colors.

PROPERTIES

General properties:
Chemistry: Al2O3
Crystal structure: trigonal
Purity - 99.99%

Physical properties:

Density: 3.98 (colorless) – 4.1 (dark-colored rubies)
Hardness (Mohs scale): 9
Tensile strength - 415 MPa
Compression strength - 180 MPa
Bending strength - 1000 MPa
Coefficient of friction with steel - 0.14
Coefficient of friction with graphite - 0.16
Coefficient of friction with graphite - 0.19

Thermal properties:

Melting point 2046°C
Limiting temperature of usage 1000 C
Optical properties:
Refraction index: 1.77

HISTORY

Since the time of the alchemists, there have been attempts to synthetically produce precious stones. Synthetic Spinel, or, more precisely, synthetic ruby, was the first gemstone reproduced by an artificial technique.

In the 19th century the first microscopic ruby crystals were created from alumina in a laboratory in 1837. By 1877, chemist Edmond Fremy had devised an effective method for commercial ruby manufacture by using molten baths of alumina, yielding the first gemstone-quality synthetic stones. The Parisian chemist Auguste Verneuil collaborated with Fremy on developing the method, but soon went on to independently develop the flame fusion process, which would eventually come to be known as Verneuil method.

One of Verneuil's sources of inspiration for developing his own method was the appearance of synthetic rubies sold by an unknown Geneva merchant in 1880s. These " Geneva rubies" were dismissed as artificial at the time, but are now believed to be the first rubies produced by sintering natural crystals.

The first results in the process of creating corundum from aluminum oxide were achieved by Verneuil in 1892. By 1910, Verneuil's laboratory had expanded into a 30 furnace production facility, with annual gemstone production having reached 1,000 kg (2,205 lb) in 1907. The most notable improvements in the process were made in 1932 by S.K.Popov, who helped establish the capability for producing high-quality sapphires in the Soviet Union through the next 20 years. A large production capability was also established in the United States during World War II, when European sources were not available, and jewels were in high demand for their military applications.

SYNTHESIS

Verneuil method utilizes the powdered ingredients of a gem by fusing them together under a high temperature oxy-hydrogen flame. The ingredient powder melts and re-crystallizes in successive layers on a support below the frame, creating a cone-shaped boule, the color of which can be controlled by addition of different chemical ingredients.

Verneuil synthesized ruby from the finely ground aluminum oxide powder mixed with chromium oxide. The mixture was melted by a flame of at least 2000 °C (3,600 °F) in a furnace and re-crystallized into a boule which was lowered as it grew bigger. The simplicity, reliability and high productivity of this method allowed to produce large quantities of corundum for commercial purposes. It is still the most inexpensive crystal production method that offers very good value for money. Consequently, it is used not only to make the majority of created rubies and sapphires, but also some other minerals, like spinels.

To achieve color variation, different chemical components can be added to the melted powder. The first attempts to create sapphire by this method failed, as added cobalt oxide which was meant to give blue color was not evenly distributed in the crystals. Moreover, the crystals had an unpleasant grey hue. Sapphire was eventually synthesized by adding ferric and titanic oxides. Other additives that are used to create corundum of different colors are manganese, titanium, vanadium and nickel. They are added in tiny quantities and often mixed to achieve a better result.

Synthetic corundum produced by means of Verneuil method is rather easy to distinguish from natural material by the presence of curved growth lines resulting from crystallization in layers and trapped spherical gas bubbles which are never seen in natural rubies and sapphires.

Other methods of growing synthetic corundum, such as flux-grown, Chokhralski pulled, and hydrothermal methods, produce more realistic imitations, which are primarily identified by characteristic inclusions. They are much more expensive than the Verneuil synthetics but considerably cheaper than the rubies and sapphires that they imitate.

Whereas Verneuil's technique is quite simple and produces large quantities of synthesized material at relatively low costs, the hydrothermal method is used to create high quality rubies and sapphires, especially since this method best imitates the natural condition of gems' formation. Hydrothermal technique involves dissolving the crystal nutrients in an acidic solution of water and chemicals at high temperatures and pressures in one part of the container called autoclave, with crystallization on a seed crystal occurring in a cooler chamber of the autoclave.

The nutrient for corundum crystals consists primarily of pure aluminum oxide (Al2O3); approximately 5-8% of chromium oxide (Cr2O3) must be added to produce the essential red color of rubies, and to produce blue sapphires titanium and ferric oxides are added to the nutrient. If a star ruby is being produced, a small amount (0.1-0.5%) of titanium oxide (TiO2) is also used. A water-based solution of sodium or potassium carbonate is used as a solvent in the hydrothermal process. A corrosion-resistant metal such as silver or platinum is used to line up the autoclave that contains the ingredients. As a seed crystal, synthetic corundum produced by Verneuil's method of Floating Zone method was used. The higher the quality of the seed crystal, the better quality rubies and sapphires are produced by hydrothermal method.

Powdered or crystalline nutrient is dissolved in a water-based solution at high temperature in the lower part of the autoclave. A seed crystal is suspended in the upper part of the autoclave, and a special baffle is used to separate the two zones. The container is then sealed shut and placed vertically in a furnace chamber, with the nutrient-containing end of the autoclave resting on a heating element. As the floor of the furnace is heated, the bottom end of the tube becomes hotter than the top (about 445° C compared to 410° C). The dissolved nutrient material migrates toward the seed and crystallizes on its relatively cooler surface. Pressure within the tube can range from 83,000-380,000 kPa, depending on the amount of free space left in the tube when the solvent was inserted. The tube used for the hydrothermal process can be made in any appropriate size, with a height-to-diameter ratio ranging from 8-16. In an example described in Synthetic Gem and Allied Crystal Manufacture, five seed crystals were placed in a 12 in (300 mm) long tube; each crystal grew at a rate of 0.006 in (0.15 mm) per day during the 30-day processing period of growing ruby.

Synthetic corundum has a wide range of industrial applications, but it is also used as a gemstone. Some of the most popular applications include the following:

High class watch glass
Optics (windows, lenses)
Laser applications
Microelectronics (e.g., substrates)
Components of machinery and equipment (e.g. laser or scanner components)
Medicine
Jewelry

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