Editor’s Note: Many gemstones can be created in the laboratory and have a longer manufacturing history than diamond. This synthetic gemstone guide covers rubies, sapphires, emeralds, opals, and many more species and explains their fabrication processes. This five-part series of articles, “Understanding Gem Synthetics, Treatments, and Imitations,” is a chapter from Dr. Joel Arem’s forthcoming book, Gems and Jewelry, 3rd Edition. © Joel E. Arem 2011-2013. The International Gem Society (IGS) gratefully thanks Dr. Arem for his contributions to the field of gemology and for allowing us to reproduce this chapter.
Ruby and Sapphire
Ruby is aluminum oxide colored red by chromium. Synthetic ruby is often made by simply melting aluminum oxide that contains a trace of chromium. The resulting crystal has the same internal atomic structure as natural ruby as well as the same optical properties, hardness, and chemical composition. In fact, the only significant difference between this material and natural ruby is the place of origin, a laboratory, rather than deep within the earth.
Ruby and sapphire have long been considered two of the most desired and valuable gems. Natural material has never been available in sufficient quantity to meet world demand. It is therefore not surprising that their synthesis would be considered a worthy goal. The earliest experiments were those of Marc Gaudin in France in the mid-19th Century, although he never achieved the creation of gem quality corundum. In the mid 1880’s, however, rubies appeared on the gem market that were initially thought to be natural, but which careful study showed to be manufactured products. Many of these rubies, known as “Geneva rubies,” because it was thought that they were made near Geneva, Switzerland, were sold as natural. Just after the turn of the century another type of ruby appeared on the market. Termed “reconstructed ruby,” this material was supposed to have been made by melting together bits of natural ruby. In recent years it has been demonstrated that such a process will not work, so these rubies must also have been synthesized from chemical raw materials.
A commercial process for manufacturing ruby was developed by Edmund Fremy of Paris. His rubies, however, all emerged in the form of thin plates. They could be manufactured cheaply in great quantity, and were sold widely for use in watch and instrument bearings. But they were too thin to provide large gems of fine color. In the last decade of the 19th Century, one of Fremy’s assistants, August Verneuil, developed a new and different technique for synthesizing 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 has already been described.
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. The furnaces can be automated so minimal staff can run many machines. Factories in Germany, France, and Switzerland may contain nearly 1,000 furnaces running at the same time, night and day. Massive production also exists in China, Thailand, and elsewhere. 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 so produced, called boules, are cut in mass-production shops, sometimes by machine or by hand where labor is inexpensive.
There are other techniques for manufacturing corundum. Ruby for lasers can be grown by “pulling” crystals from a melt (Czochralski method), which can yield single transparent crystals inches across and several feet long. 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 far more expensive than the Verneuil process. The flux process for ruby was perfected decades ago by Judith Osmer, but her trademarked “Ramaura” ruby is unfortunately no longer available in the marketplace.
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 actually synthetic corundum that has a color change reminiscent of true alexandrite. Colorless corundum, 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 and military applications.
Synthetic Corundum Gemstones Grown By Flame Fusion Method
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), producing a six-rayed star when cabochon 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 and glass 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.
The first synthetic spinel was produced accidentally when some magnesium oxide was added to the feed powder in making synthetic Verneuil corundum. Spinel was not considered an especially valuable gem, however, so more than 20 years passed before synthetic spinel was used commercially in quantity. Natural spinels are not commonly encountered in the gem trade, but synthetic spinels are seen almost everywhere. These gems are widely used to imitate other gems that are considered more desirable, such as emerald, aquamarine, and tourmaline.
Synthetic spinel is normally made by the Verneuil process, and boules in a tremendous variety of rich colors can be grown. These colors are due to the addition of chemical impurities because pure spinel, as with pure sapphire, is colorless. In addition, spinel powder mixed with cobalt oxide and fused in an electric furnace produces a dense, deep-blue material that strongly resembles lapis lazuli. A spinel that resembles moonstone was introduced in 1957. Some spinel has also been made by flux fusion, and this material can be difficult to distinguish as synthetic.
Synthetic spinels may not show the curved growth lines seen in synthetic Verneuil corundum. But they can usually be identified as spinel (by refractive index), and the colors of the synthetic gems are usually sufficiently different from those of natural stones to warrant further testing.
Natural quartz is common and inexpensive. Yet synthetic quartz can be made in sufficient quantity and at low enough cost to make gem quartz manufacture worthwhile. Citrine, or yellow quartz, is colored by iron. Amethyst is made by adding specific impurities that produce a brownish color. A purple hue is created when this quartz is irradiated by a radioactive source. Colorless quartz is made in ton quantities for use in electronic applications but is seldom cut as a gem. Green quartz is also manufactured in limited quantity. Quartz is synthesized by the hydrothermal method. This is the way most natural mineral crystals form, in veins and cavities within the earth. While natural solutions are very dilute, and mineral crystals may take many years to form, in the laboratory the action is dramatically sped up.
Of the various beryl colors, by far the most valuable is the deep green of emerald. Experiments at emerald synthesis are known as early as 1848, but crystals weighing more than one carat could not be synthesized until 1912. Richard Nacken, who also developed the basic process for quartz synthesis, produced small emerald crystals using a hydrothermal process similar to that used for quartz. Later German experimenters succeeded in growing small emeralds of fine color, which were marketed as “Igmerald” by the I. G. Farbenindustrie conglomerate as early as 1934.
Synthetic Emerald (Gilson)
After World War II, Carroll Chatham of San Francisco introduced emeralds of large size and fine color. These were the result of research dating back to 1930, and apparently were made using a flux-growth technique. Synthetic emeralds have also been manufactured by the Linde Air Products Company, Pierre Gilson of Paris, Zerfass of Germany, and many others. The Linde emerald is grown hydrothermally using seed plates of colorless beryl. Gems are cut from the emerald that accumulates above or below the seed plate, so large thicknesses are required and are expensive to prepare. Large crystals of superb color are made by Gilson, and clusters of synthetic crystals are frequently offered for sale as jewelry items.
Synthetic emeralds can usually be distinguished from natural gems by the presence of characteristic inclusions. Natural emeralds have specific kinds of inclusions, which are often diagnostic of the country or mine of origin. Sometimes present are so-called “three-phase” inclusions consisting of a cavity filled with liquid, with a gas bubble and a crystal of sodium chloride or another salt inside. Synthetic emeralds do not generally display such inclusions, but may contain pieces of flux or other characteristic internal markings. Detection always requires the use of a microscope and, sometimes, additional gemological testing instruments.
Other Synthetic Gems
Pierre Gilson of Paris introduced three remarkable synthetic gems: opal, turquoise, and lapis lazuli. It is now known that the color flashes in precious opal are due to the regular accumulation of layers of minute spheres. Gilson duplicated this process in the laboratory, and his synthetic black and white opal is spectacular and natural looking. Careful tests may be required to distinguish it from natural opal.
Gilson turquoise resembles the finest Persian turquoise. It is extremely uniform in color and texture and available in cut stones or rough blocks. Under the microscope this turquoise consists of an aggregate of tiny spheres of uniform size, allowing it to be readily distinguished from natural turquoise.
Synthetic alexandrite is not corundum with an alexandrite-like color change, but rather a true synthetic chrysoberyl containing suitable impurities. The color change is green to red, resembling Russian alexandrite. Large cut gems are available, but the cost is high for a synthetic – in the range of synthetic emerald.
Synthetic rutile, chemically titanium oxide, appeared on the market in 1948, under various trade names. Natural rutile is nearly always opaque or a very dense, deep red color. Synthetic rutile can be made by the Verneuil process in a variety of colors, including brown, yellow, red, and blue. Completely colorless stones cannot, however, be produced and always have a tinge of yellow. The colored varieties were seldom seen in the gem trade. Rutile is too soft to be useful as a gemstone (hardness 6-6.5 on the Mohs scale). But its dispersion is about six times higher than that of diamond. Cut rutile therefore blazes with myriad colors. The color display is so dazzling and breathtaking that cut rutile loses credibility as the diamond it is supposed to imitate. There is simply too much fire to be “real.” Cut rutile, sold as “Titania,” is occasionally still available, but long ago lost its popularity to more suitable diamond imitations, especially cubic zirconia.
Some other synthetic materials that have natural analogs include: scheelite (calcium tungstate); apatite (calcium phosphate); wulfenite (lead molybdate); proustite (silver arsenic sulfide); gahnite (zinc aluminate, a variety of spinel); periclase (magnesium oxide); fluorite (calcium fluoride); zincite (zinc oxide); bromellite (beryllium oxide); feldspar (aluminum silicate); zircon (zirconium silicate); phenakite (beryllium silicate); and sphalerite (zinc sulfide). All of these have probably been cut as curiosities for gem collectors.