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, Czochralski 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 synthesised 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.

#78 I Grade (no skin)	#78 II Grade (no skin)	#78 II Grade (with skin)

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 crystalizes 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.

PROPERTIES

General properties:

            Chemistry: Al₂O₃

                Crystal structure: trigonal

Purity - 99.99%

Physical properties:

            Density: 3.98 (colourless) – 4.1 (dark-coloured 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

Synthetic corundum has a wide range of industrial applications, but it is also used as a gemstone