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Saturday, 02 April 2011 21:03

Synthetic Gems

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Synthetic gems are chemically and structurally identical to stones found in nature. Imitation gems, in contrast, are stones that are made to appear similar to a particular gem. There are a few basic processes that produce a variety of gem stones. Synthetic gems include garnet, spinel, emerald, sapphire and diamond. Most of these stones are produced for use in jewellery. Diamonds are used as abrasives, while rubies and garnets are used in lasers.

The first synthetic gem used in jewellery was emerald. The process employed in its manufacture is proprietary and kept secret, but probably involves a flux-growth method in which silicates of alumina and beryllium with additions of chromium for colour are melted together. Emeralds crystallize out of the flux. It may take a year to produce stones by this process.

The Verneuil or flame-fusion process is used in the production of sapphire and ruby. It requires large amounts of hydrogen and oxygen, therefore consuming great amounts of energy. This process involves heating a seed crystal with an oxyhydrogen flame until the surface is liquid. Powered raw material such as AI2O3 for sapphire is added carefully. As the raw material becomes molten, the seed crystal is slowly withdrawn from the flame, causing the liquid furthest from the flame to solidify. The end closest to the flame is still liquid and ready for more raw material. The end result is the formation of a rod-like crystal. Sundry colours are created by adding small amounts of various metal ions to the raw materials. Ruby is created by replacing 0.1% of its aluminium ions with chromium atoms.

Spinel, a colourless synthetic germ (MgAI2O4), is made by the Verneuil process. Along with sapphire, spinel is used by industry to provide a wide range of colours for use as birth stones and in class rings. The colour produced by adding the same metal ions will be different in spinel than it will be in sapphire.

Synthetic diamonds are used in industry because of their hardness. Applications for diamonds include cutting, polishing, grinding and drilling. Some of the common uses are cutting and grinding of granite for use in building construction, well drilling and grinding non-ferrous alloys. In addition, processes are being developed that will deposit diamond on surfaces to provide clear, hard, scratch-resistant surfaces.

Diamonds are formed when elemental carbon or graphite is subjected to pressure and heat over time. To create a diamond on the factory floor involves combining graphite and metal catalysts and pressing them together in high heat (up to 1,500 °C). The size and quality of the diamonds are controlled by adjusting the time, pressure and/or heat. Large tungsten carbide dies are used to achieve the high pressures needed to form diamonds in a reasonable period of time. These dies measure up to 2 m across and 20 cm thick, resembling a large doughnut. The mixture of graphite and catalyst is placed in a ceramic gasket, and tapered pistons squeeze from above and below. After a specified time, the gasket containing diamonds is removed from the press. The gaskets are broken away and the diamond-bearing graphite is subjected to a series of agents designed to digest away all material except for the diamonds. The reactants employed are strong agents that are potential sources of significant burns and respiratory injury. Gem-quality diamonds may be produced in the same manner, but the long press times required make this process prohibitively expensive.

Hazards resulting from the manufacture of diamonds include potential exposure to the highly reactive acids and caustic agents in great volumes, noise, dust from forming and breaking of ceramic gaskets, and metal dust exposure. Another potential hazard is created by the failure of the massive carbide dies. After a variable number of uses, the dies fail, posing a trauma hazard if the dies are not isolated. Ergonomic issues arise when the diamonds manufactured are classified and graded. Their small size makes this a tedious and repetitive job.



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