Aluminium oxide This is the most common synthetic product used as a substitute for natural gems, and it has been widely employed for technical purposes as well; for example, for the jewels in watches.
Trigonal, like the corresponding mineral In most cases, the synthetic product does not have well-developed crystallographic faces.
The vast majority of synthetic corundums produced by the Verneuil flame fusion method are pear-or sausage-shaped, thinning down into a short peduncle at one end. Specimens produced by other methods of fusion (mainly the Czochralsky process, also known as pulling from the melt are uncommon and have a squat cylindrical shape with horizontal striations, terminating in a cone at one end, or a very long, cylindrical, rod shape. But the most recent (costly and therefore little used) flux melt and hydrothermal processes produce crystals, singly or in groups, which are very similar to the natural ones. A complete range of colors, as well as colorless, can be obtained. Colors may be red to pink, orange, yellow, green, blue violet and gray-green turning to violet red.
Identical to those of natural corundum: hardness 9; density about 4.0 g/cm3; aacretractive in-dices about nE 1.760, rko 1.769
this is concentrated in certain countries with highly developed chemical industries, notably Switzerland, France, Germany Italy Czechoslovakia the Soviet Union, Japan, and the United States.
This was the first synthetic gem to be manufactured on an industrial scale and quantities have steadily increased to the present day, the Verneuil method being the most widely used.
Synthetic ruby is usually bright red, differing very little from the natural stone, the physical proper-ties of which are also faithfully reproduced. Ills given the same oval, round or pear shaped mixed faceted cuts or is made into cabochons. But it is also cut into special shapes, often weighing 5-15 carats; for instance, rectangular with a smooth, convex upper surface and faceted lower sur-face, or with the top part convex, but consisting of numerous, juxtaposed square facets and the bottom part faceted; or oval, again with a smooth, convex upper surface and faceted lower surface. These cuts are characteristic of synthetic rubies. They are found in large stones that are highly transparent, being completely free of inclusions, and are often used for large, old-fashioned rings for men or set into religious objects. Beads 2-3 to 7-8 millimeters in diameter are also typical; of a perfectly even, bright red color and uniform diameter, they are made into necklaces and brace-lets. These pieces of jewelry could not possibly be made of natural rubies. On the rare occasions when natural rubies are used for this purpose, they are generally of mediocre color, full of inclusions, and of graduated diameters. Fine quality natural ruby is much too valuable to be treated in this way.
These are almost exclusively internal generally speaking, this material is quite limpid. Provided one can find the right direction, thin (noncrystallographically oriented) curved lines, characteristic of growth by deposition of successive layers of molten material, will be visible under a lens, or better still a microscope, and sometimes, small gas bubbles and "swarms" of minute, opaque foreign bodies (unmelted alumina powder) can be seen. In the past, a network of internal cracks was sometimes produced by a sudden change of temperature to compensate for the suspicious absence of inclusions. The resulting stones appeared vaguely similar to some natural rubies with numerous inclusions.
Extremely low, bearing no relation to that of their natural counterparts; in fact, most of the cost of ordinary synthetic rubies is in the cutting. The rare synthetic rubies produced by the flux melt or hydrothermal processes are much more expensive, costing little less than the better secondary gems. Hence, manufacture of these is not normally economical. But given their resemblance to natural stones and the possibility of some of them being sold as "good," there is a market for them. While sale of these stones invariably starts out perfectly above board, it sometimes ends in a highly profitable fraud after a few changes of hands, because of the high value of natural rubies of similar characteristics.
Production of this synthetic gem started a few years later than ruby, greater difficulties being encountered in reproducing the color.
The color of medium-light faceted stones often looks darker at the edges, due to an optical effect. It may also be colder and grayer than natural sapphires. It is cut into all the shapes used for the natural stones, both faceted and cabochon. The cabochon cut is, in fact, the one that best suits it. Synthetic sapphires cut en cabochon are the most convincing and hardest to distinguish from natural stones.
The color, sometimes with an un-usual shade and color zoning, and the absence of blue-green pleochroism may distinguish synthetic sapphire from some, but not all, natural sapphires, given their variability. Here, too, the main distinctive features are internal and only visible under a lens or microscope: broad band’s rep-resenting curved growth lines emphasized by a different depth of color are much more clearly visible than those of synthetic rubies if the stone is examined against the light. Sometimes gas bubbles and minute foreign bodies either separately or in swarm follow the growth curves. Cost As with synthetic ruby, the cost is very low and mainly accounted for by the cutting process.
The varieties most often seen are colorless, pink, various shades of yellow including brown- or orange-yellow, and violet. Rarer are gray-green stones that turn reddish in artificial light. The colorless variety was used in the past to imitate diamond; the pink is a good imitation of pink sapphire; the yellows have mainly been used to imitate topaz, although they are not very similar; and the amethyst violet variety is normally for some inexplicable reason called synthetic alexandrite, despite the fact that it looks quite different from alexandrite chrysobery The variety which changes color is intended to be an imitation of alexandrite, but it is not a very convincing one. These stones are given more or less all the types of cut used for colored stones, particularly those they are designed to imitate.