Antique jewelry glossary

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Synthetic gemstone

An artificial, man-made stone used in the same manner as a natural gemstone, having the same appearance, chemical composition, and physical characteristics, including crystalline ctructure, specific gravity, refractive index, colour dispersion, and hardness. Such stones can be made colourless, or, by the use of metallic oxides, in many colours, and thus can be made to resemble many natural stones, including the diamond, spinel, emerald, and corundum (ruby and sapphire). Although experiments in making synthetic rubies were successfully concluded in 1877 by the French chemists Edmond Frémy and Charles Feil, who produced synthetic corundum, it was not until the invention of the Verneuil furnace in 1904, and its production of the boule, that commercial production became feasible.

Today many synthetic gemstones are mass produced. Some man-made stones have no counterparts in nature and so are not strictly synthetic stones but rather are oddities (e.g. strontium titanate; lithium niobate; yttrium aluminium-garnet; cubic zirconia); however, they are generally considered as synthetic gemstones except when presented to imitate a natural gemstone, in which case they are considered a simulant.

Synthetic gemstones are usually sold by size in millimetres, rather than by weight. Testing to distinguish a natural stone from a synthetic stone is usually done by microscopic examination, as the minute internal inclusions often vary between the natural stone and its synthetic counterpart. A synthetic stone should be designated preferably, not by a coined trade-name, but by the name of the stone that it simulates, preceded by the word 'synthetic'. It must be distinguished from an imitation gemstone.

Synthetic alexandrite

A misnomer for a synthetic gemstone that is either a synthetic corundum (sapphire) coloured bluish-green by oxide of vanadium, or a synthetic spinel, both of which resemble alexandrite. Such stones were once miscalled 'scientific alexandrite' owing to their having the property of changing colour in the manner of alexandrite. The synthetic corundum has been sold under the trade-name of 'Syntholite'. An experimental synthetic chrysoberyl has been produced as a true synthetic alexandrite.

Synthetic beryl

A synthetic gemstone resembling the beryl that is coloured green by use of vanadium oxide.

Synthetic corundum

A synthetic gemstone that has been made with many colouring agents, producing stones in many hues and shades. A colourless variety, produced with pure alumina free from potash, correspond to white sapphire and has been called 'Walderite'. Another variety simulates the alexandrite; a green variety has been called 'Amaryl'; and a yellow variety simulates topaz. The earliest synthetic corundum was the synthetic ruby, next the synthetic sapphire. Synthetic corundum stones can be produced as asterias (star stones), to simulate both star ruby and star sapphire by the addition of titanium oxide which, at high temperature, is precipitated as needles of rutile. Such stones can be distinguished from natural stones by use of a microscope which will reveal internal curved striae, and also the presence of tiny spherical gas bubbles (although such bubbles may be minimized by modern processes); more exact tests require use of ultraviolet light and X-rays. Synthetic ruby and synthetic sapphire are now produced by other methods, and detection is more complicated.

Synthetic diamond

A synthetic gemstone that resembles the natural diamond, produced at very high temperature and under great pressure. The earliest experiments were made before 1880 by a Glasgow chemist, J.B. Hannay, who produced some small stones of doubtful authenticity. Experiments by F.F.H. Moissan in the 1890s and by others later proved inconclusive. In 1955 a successful process was developed using techniques devised by Dr Percy W. Bridgman (1882-1961) for the General Electric Co. in the United States, producing by high temperature and enormous pressure some very small stones (1,2mm in length). As a result, this firm and others in Sweden and elsewhere now produce quantities of small synthetic diamonds for industrial use, but they are not large enough for gemstones and are more costly than natural stones.

By 1970 General Electric Co. had produced gem-quality synthetic diamonds that were colourless or that showed yellow or blue, but they show technical deviations from the natural stones and under a microscope have a 'dusty' internal look. Some man-made stones are sold as simulants of diamonds but are not synthetic diamonds, e.g. cubic zirconia; yttrium-aluminium-garnet, and strontium titanate.

Synthetic doublet

A composite stone consisting of a piece of red synthetic spinel or synthetic corundum cemented alongside a piece of blue synthetic corundum, forming a pseudo-parti-coloured stone.

Synthetic emerald

A synthetic gemstone resembling the natural emerald, produced by several processes. Early experiments by P.C. Hautefeuille and A. Perrey, c. 1890, produced stones too small to be cut as gemstones, and later attempts by use of the Verneuil Furnace failed because the substance fused as glass rather than in a crystalline form. The first success came in 1928 by use of the hydrothermal process by E. Nacken, of Frankfurt. Then in 1934 a synthetic emerald of commercial size, called Igmerald, was produced in Germany by the I.G. Farbenindustrie, using the flux fusion process, followed in 1935 by C.F. Chatham in San Francisco, in 1963 by W. Zerfass in Germany and by Pierre Gilson in France, and in 1965 by the Linde Co. of California. These and other synthetic types later produced can be distinguished from natural stones by their being lighter and having lower refractive index and specific gravity, as well as by the use of a colour filter; but more effective tests are by microscopic examination which reveals in the synthetic stones the character of the feathers (two-phase inclusions, rather than the three-phase inclusions of a true emerald) and by tests for fluorescence which is strong in the synthetic stones.

Synthetic fluorspar

A synthetic gemstone that resembles natural fluorspar and is artificially coloured. As is the case with the natural stone, it is too soft to be serviceable for jewelry.

Synthetic garnet

A misnomer for a synthetic stone that has no counterpart in nature but is a compound synthesized by the Czochralski process ( a variation of the flux fusion process); it is not a silicate as is a natural garnet, but has a similar structure. One colourless variety has been called YAG (for yttrium-aluminium-garnet) or 'Diamonair'; it simulates the diamond. Other varieties are made in many colours. They have single refraction but can be distinguished from a natural garnet by several tests.

Synthetic periclase

A synthetic gemstone that resembles the natural periclase. It is almost colourless and has been produced by the flame fusion process with the Veneuil Furnace. A trade-name for one variety is 'Lavernite'.

Synthetic ruby

A synthetic gemstone resembling the natural ruby. The first synthetic rubies were attempted by Marc Gaudin in 1837, and by 1877 thin platy crystals of ruby were produced by the French chemists Edmond Frémy and Charles Feil; later Frémy and Auguste Verneuil succeeded in producing rubies large enough to be rose cut, but too small for general use as jewelry. A so-called reconstructed stone having the appearance of a ruby was produced c. 1885. Successful results from the Verneuil Furnace process, using as a colouring agent oxide of chromium, were published in 1904, and today synthetic rubies (including the synthetic pigeon's-blood ruby) are commercially produced by modifications of that method, and also by the advanced hydrothermal process and flux fusion process. Synthetic rubies can be distinguished by the often 'too perfect' colour, by curved rather than straight striae, and by the typical internal bubbles, and sometimes by ultraviolet and X-ray tests.

Synthetic rutile

A synthetic gemstone of the same composition as natural rutile but of different appearance. It has been produced since 1948 by a modification of the flame fusion process of the Verneuil Furnace. The resulting boules are opaque black, but when reheated in a stream of oxygen they change to various colours. The variety most in demand commercially is pale yellow to colourless, resembling a diamond but distinguishable by its colour, greater fire, much higher colour dispersion, lower hardness, and having strong double refraction. It has been sold under many trade-names, e.g. Titania, Titangem, Diamothyst, Titanstone, etc., or the misnomer 'Rainbow diamond'. It has been largely superseded by synthetic strontium titanate which is whiter.

Synthetic sapphire

A synthetic gemstone resembling a natural sapphire, first produced by the Verneuil Furnace in 1910. Early experiments had used cobalt oxide, but the colour concentrated in patches, so magnesium oxide was added as a flux; this resulted in a stone that was a synthetic spinel rather than a synthetic sapphire. Later magnetic oxide of iron and titanic acid were added, and a clear transparent blue synthetic sapphire was produced. The synthetic star sapphire was produced by adding more titanium oxide, which precipitated to form needles of rutile. A colourless variety (misleadingly called walderite) has been produced by use of pure alumina, and various coloured varieties by use of different metallic oxides, e.g. with nickel oxide a pinkish-orange stone and with vanadium oxide a greenish stone resembling alexandrite. The synthetic varieties can be distinguished by their showing, when viewed through a microscope, curved colour bands as opposed to straight bands, and sometimes minute gas bubbles.

Synthetic spinel

A synthetic gemstone that resembles the natural spinel. It was first produced accidentally in experiments to produce a synthetic sapphire by the Verneuil Furnace process and using cobalt oxide and also magnesium oxide as a flux. It can be made now to imitate gemstones of several other species, e.g. ruby, sapphire, diamond, zircon, tourmaline, aquamarine, alexandrite, etc. The synthetic stones can generally be distinguished by lack of curved growth lines (striae), by colour banding, and by crystal inclusion, and also by occasional internal globular gas bubbles, as well as by slightly different colour, refractive index, and specific gravity. Other varieties of synthetic spinel include an imitation of lapis lazuli (produced by sintering powdered colourless synthetic spinel and cobalt oxide) and of moonstone (produced by applying heat treatment to a colourless synthetic spinel). There is a synthetic blue spinel that can be distinguished from natural blue spinel by use of a colour filter, and a synthetic red spinel that resembles ruby but is distinguished by its fluorescence.

Synthetic turquoise

A misnomer for an imitation gemstone made of blue glass to simulate turquoise. An actual synthetic turquoise has now been produced.

From: An Illustrated Dictionary of Jewelry, autor: Harold Newman, publishers: Thames and Hudson