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Synthetic Diamond

In the last decade of the 19th Century, one of Fremy's assistant-August Verneuil, developed a new and different technique for synthe¬sizing ruby. Fremy's method involved dissolving aluminum oxide in a molten salt, and allowing ruby to crystallize from the melt by slow cooling.
Verneuil's method, which he called "flame fusion," employs the direct melting of aluminum oxide in a flame. The powdered chemi¬cal is allowed to dribble from a hopper through a very hot flame.
 

The powder melts in the flame and falls in the form of tiny droplets onto a rotating ceramic rod. Eventually a mass of material builds up which cools and crystallizes as a large single crystal. Ruby can be made by adding a pinch of chromium to the aluminum oxide. Sapphire in various colors requires different combinations of metal oxides. It is interesting that the basic design of the Verneuil furnace hasn't changed much since the day it was first introduced in 1904.

 

Synthetic Diamond

Synthetic Diamond

Most synthetic gem ruby and sapphire today is grown by the Ver¬neuil process. The furnaces can be automated so a minimum of person¬nel can run many machines. Factories in Germany, France, and Switzer¬land may contain nearly 1,000 furnaces running at the same time, night and day. The output of such factories is measured in tons, rather than carats, and the cost of rough synthetic corundum can be as low as a few cents per carat. The crystals produced, called boules, are cut in mass ¬production shops, sometimes by machine, or by hand where labor is inexpensive.

A few companies use other techniques for manufacturing corun¬dum. Ruby for lasers is grown by pulling crystals from a melt„in a way reminiscent of pulling taffy, although the procedure is complex and carefully controlled. A more refined version of Fremy's method is also used to a limited extent. Today the method is called flux fusion, and the process yields ruby of fine color and clarity, although it is tar more expensive than the Verneuil process.

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Synthetic sapphire and ruby appear in a variety of commercial jewelry, such as class rings and birthstone jewelry. Usually a ring sold as "alexandrite" or "amethyst," where the label includes the quotes, is a synthetic stone. The so-called "alexandrite" sold to tourists throughout the world for a few dollars per stone is a specially made corundum that has a color change reminiscent of true alexandrite. Colorless corundum, 138 or "white sapphire" is manufactured in huge quantities for use as colorless gems and for bearings in electric meters, as well as for use in specialized electronic applications.

Star ruby and sapphire can be made by adding titanium oxide to the feed powder in a Verneuil furnace. As the corundum cools, the titanium oxide forms crystals of the mineral rutile within the host corundum. The rutile crystals are needle-like and orient themselves according to the symmetry of the corundum, which is hexagonal (six-sided). This pro¬duces a six-rayed star when such boules are cut. The color range of synthetic star corundum is the same as that of the faceted gems.

Synthetic corundum has distinguishing characteristics. The Verneuil process always produces curved growth lines which are visible under magnification and with the correct illumination. No natural mineral ever displays such curved lines, called striae, and their presence is a guarantee of synthetic origin. Another characteristic of synthetics is the presence of perfectly round bubbles, sometimes with a small tail like a tadpole. Flux-grown rubies may show characteristic inclusions of the flux. Other tests normally used in gem identification may not be helpful.


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