Tuesday, 30 July 2013

Synthetic Corundum

Synthetic Corundum
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.


Crystal system

Trigonal like the corresponding mineral, In most cases, the synthetic product does not have well-developed crystallographic faces.

Appearance

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. Physical properties Identical to those of natural corundum: hardness 9; density about 4.0 g/cm3

Production

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.

Synthetic ruby

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.

Appearance

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 surface, or with the top part convex, but consisting of numerous, juxtaposed square facets and the bottorn 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.


Distinctive features

Synthetic ruby These are almost exclusively internal: generally speaking, this material is quite limpid. Provided one can find the right direction, thin (noncrystal lographically 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.

Cost

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 some-times ends in a highly profitable fraud after a few changes of hands, because of the high value of natural rubies of similar characteristics.

Synthetic sapphire

Production of this synthetic gem started a few years later than ruby, greater difficulties being encountered in reproducing the color. Appearance 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.


Distinctive features

The color, sometimes with an unusual 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 bands representing 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 ' 'swarms" follow the growth curves. Cost As with synthetic ruby, the cost is very low and mainly accounted for by the cutting process.

Synthetic star ruby and sapphire

Synthetic ruby
This variety of corundum has also been manufactured synthetically for a few decades. Titanium oxide is added to the alumina. It is then precipitated out as tiny crystals along the corundum crystal lattice by a process of very slow cooling, giving rise to the star effect.

Appearance

These stones are invariably cut into round or oval cabochons, which generally have a star with six very obvious, if thin, rays. They normally have very limited transparency. The usual colors are bright red (ruby), or equally bright blue (sapphire), although blue-gray and dullish red stones have been produced.


Distinctive features

Synthetic CorundumThe star is very obvious and clear-cut, more so than in most natural gems. The curved growth lines characteristic of synthetic corundum are nearly al-ways clearly visible. Sometimes, these are faintly visible in the form of concentric circles on the unpolished lower surface of the stone as well. One of the leading manufacturers also used to engrave the company's initial, a squat letter L," on the underside of cabochons.

Cost

Low, although a bit higher than that of normal synthetic rubies and sapphires, as the method of producing them is more complex.


Colorless and other varieties of synthetic corundum

Early attempts with ruby proving relatively straightforward synthetic corundum soon began to be produced in many different colors, not so much to imitate other types of natural corundum (some of these colors do not occur naturally), as to provide a highly effective, inexpensive ornamental material or even to imitate gems of quite a different mineralogical nature.


Appearance

Colorless and other varieties of synthetic corundum The varieties most often seen are colorless, pink, various shades of yellow including brown or orange-yellow, and violet. More rare 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 chrysoberyl 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, but the round, mixed cut is more often seen than with natural stones. These stones are often quite large (easily 5-15 carats or more), except for the colorless variety, which generally appears in small stones, which are harder to distinguish from diamond. All the varieties of synthetic corundum have the characteristic fine luster of natural corundum, although this is not always shown to advantage in poorly cut stones.

Distinctive features

These synthetic corundums, which are all produced by the Verneuil method, usually display characteristic growth curves, although these may be barely visible, if at all, in the yellow and orange-yellow varieties. They lack, of course, the typical inclusions of their natural counterparts. Where they are used to imitate a gem of a different mineral type, this can immediately be detected by measurement of the physical properties.

Cost

Very low even for fine specimens.

Writer - Curzio Cipriani & Alessandro Borelli
 

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