Synthetic Ruby

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Synthetic Ruby

Background

Diamonds, rubies, sapphires, and emeralds are known as precious gems. Next to the diamond, the ruby is the hardest gemstone; it is also resistant to acids and other harmful substances. Because large, gem-quality rubies are very rare, the value of a fine ruby may be quadruple that of a similar-quality diamond.

Rubies and sapphires are both composed of corundum, which is the crystalline form of aluminum oxide. They differ only in small amounts of color-producing minerals. Chromium gives rubies their characteristic red color, with higher concentrations producing darker shades. Aluminum oxide crystals not containing chromium are called sapphires; they come in many hues including blue, yellow, green, pink, purple, and colorless.

Natural rubies are found in a handful of sites around the world, most notably in Myanmar (formerly Burma), Thailand, Sri Lanka, Afghanistan, Tanganyika, and North Carolina. Beautifully colored, transparent crystals are prized for jewelry use, while translucent or opaque stones are used for ornamental items such as clock bases.

In addition to their decorative functions, rubies serve a broad range of utilitarian purposes. For example, because of their hardness, they make long-lasting thread guides for textile machines. Ruby is even harder than steel, so it is an excellent bearing material for metal shafts in devices like watches, compasses, and electric meters. Rubies have exceptional wave-transmitting properties for the range from short, ultraviolet wavelengths through the visible light spectrum to long, infrared wavelengths. This makes them ideal for use in lasers and masers (laser-like devices operating in non-visible ranges of microwaves and radio waves).

Because many of these industrial uses demand very high-quality crystals of particular sizes and shapes, synthetic rubies are manufactured. With the exception of minor amounts of impurities, synthetic gems have the same chemical, physical, and optical properties as their natural counterparts. Although some are used as gemstones, about 75% of modern synthetic ruby production is used for industrial purposes.

History

Natural rubies have been mined for 8,000 years or more. In many cultures, the gems have been prized not only for their beauty but also for supernatural powers; it was commonly believed that the ruby's red color came from fire trapped inside the stone. Ancient Hindus believed that rubies could make water boil, and early Greeks thought the crystals could melt wax. In other cultures (e.g., Burmese and Native American), the ruby was thought to protect a wearer because of its blood-like color.

Because it was so highly prized, the ruby was the first gemstone to be made artificially. Documented attempts to make rubies date to the experiments of Marc A. Gaudin, a French chemist who produced some synthetic rubies beginning in 1837. They were not of any value as gems, however, because they became opaque as they cooled. After 30 years of experimenting he gave up, admitting defeat in the published notes of his final ruby experiments.

Around 1885, some rubies sold as gemstones were discovered to be manmade (their unusually low price prompted the buyer to have them carefully examined). The method by which these so-called Geneva rubies were made remained a mystery until about 1970, when an analysis of surviving samples showed that they were formed by melting powdered aluminum oxide and a smaller amount of chromium oxide in an array of torches, and letting the molten material solidify.

Actually, the Geneva rubies may have come from an early developmental stage of what is now known as the "flame fusion" method. In 1877, the French chemist Edmond Frémy and a student assistant described how they heated 44.1-66.15 lb (20-30 kg) of a solution of aluminum oxide dissolved in lead oxide in a porcelain vat for 20 days. As the solvent evaporated and chemical reactions took place among the solution, the vessel, and furnace gases, a large number of very small ruby crystals formed on the basin's wall. The rubies were so small and the production costs so high that the crystals could not realistically be used in jewelry.

Later, Auguste Verneuil, another of Frdmy's students, developed a somewhat different process that eventually became successful. By 1891 he was producing rubies by flame fusion, although he did not publish a description of his technique until 1902. His assistant exhibited the synthetic rubies in 1900 at the Paris World's Fair, where they were quite popular. His process took only two hours to grow crystals weighing 12-15 carats (2.5-3 g); the stones were roughly spherical, up to 0.25 in (6 mm) in diameter. By the time Verneuil died at the age of 57 in 1913, the process he had invented was being used to manufacture 10 million carats (2,000 kg, or 4,400 lb) of rubies annually.

In 1918, J. Czochralski developed a different method for synthesizing rubies. Known as crystal pulling, this technique is fast, inexpensive, and effective in producing flawless stones. In fact, when cut as gems the stones are so clear that they look like glass imitations. Consequently, this technique is now used primarily for manufacturing industrial-use rubies.

During World War II, it was impossible to get rubies from traditional sources in France and Switzerland. Because these stones were vitally important for use as bearings in military as well as civilian instruments, efforts were made to improve manufacturing techniques. One such improvement, developed by the Linde Division of Union Carbine Corporation, modified Verneuil's flame fusion process to grow thin rods of ruby crystals up to 30 in (750 mm) long. Such rods can easily be sliced into disks to produce large quantities of bearings.

A process developed by Bell Telephone Company in 1958 employed high temperatures and pressures to grow rubies on seeds that had been produced by flame fusion. Refinements of this technique became known as the hydrothermal method. Carroll Chatham, a San Francisco gem manufacturer who developed and used a hydrothermal process, also developed the first commercially successful application of the flux process of ruby manufacture. This technique, first used in 1959, essentially creates roiling magma in a furnace and grows very natural-looking gems in a period of nearly a year.

Methods of Synthesizing

Several methods are currently used to manufacture rubies; each has advantages and limitations. The most popular methods can be categorized into two main types: production from melt, in which powdered material is heated to a molten state and manipulated to solidify in a crystalline form, and production from "solution," in which the required aluminum oxide and chromium are dissolved in another material and manipulated to precipitate into a crystalline form. Verneuil's flame fusion and Czochralski's crystal pulling are the most commonly used melt techniques, while flux growth and hydrothermal growth are the most popular versions of solution processes.

Flame fusion rubies, generally the least expensive, are commonly used for bearings and relatively mundane jewelry like class rings. Pulled rubies, selling for upwards of $5 per carat, are preferred for laser use. Flux rubies, costing $50 or more per carat, are used in finer jewelry. The less-common hydrothermal process is used for industrial applications demanding strain-free crystals or large crystals in something other than a rod shape.

Raw Materials

The nutrient (material that will become the ruby crystal) consists primarily of extremely pure aluminum oxide (Al₂O₃); approximately 5-8% of chromium oxide (Cr₂O₃) must be added to produce the essential red color. If an asteriated gem (a star ruby) is being produced, a small amount (0.1-0.5%) of titanium oxide (TiO₂) is also used.

Depending on the method being employed, additional chemicals may be needed. The flame fusion process uses an oxygen-hydrogen torch to melt powdered forms of the two basic components, whereas the Czochralski process uses some form of electrical heating mechanism. The flux method uses a compound such as lithium oxide (LiO), molybdenum oxide (MoO), or lead fluoride (PbF₂) as a solvent for the nutrient. The hydrothermal process uses as a solvent an aqueous (water-based) solution of sodium carbonate (Na₂CO₃). A corrosion-resistant metal such as silver or platinum is used to line the vessel that contains the liquefied ingredients for the Czochralski, flux, and hydrothermal processes.

The Manufacturing
Process

Crystal growth

One of the following four methods is typically used to manufacture synthetic rubies.

1(Flame Fusion) A fine powder of the aluminum and chromium oxides is placed in a hopper at the top of the Verneuil apparatus. A hammer atop the apparatus strikes the hopper repeatedly; each stroke causes a small amount of powder to fall through the fine mesh that forms the hopper's floor. This discharged powder falls into a stream of oxygen that carries it down to a nozzle where it mixes with a stream of hydrogen and is ignited. The intense heat of this flame (around 3,600° F or 2,000° C) melts the nutrient, which falls onto a ceramic pedestal below the flame. Initially, the hammer taps at a rate of 80 beats per minute; after a suitable base for the crystal is formed, the rate is decreased to about 20 beats per minute.
After the base is built up to the desired diameter (about 0.8 in or 20 mm) and formation of the high-quality crystal proceeds, the pedestal is lowered at a rate that just keeps the top of the crystal in contact with the flame. After about five and a half hours, the crystal reaches a length of approximately 2.75 in (70 mm); the gas flow is halted, extinguishing the flame. The crystal, now weighing around 150 carats, is allowed to cool in the enclosed furnace.
2 (Czochralski Process) The nutrient is heated well above its melting point in a crucible that is surrounded by an electric heater. A small ruby crystal is attached to a rod; the desired crystal will grow on this socalled seed crystal. The seed is lowered into the crucible until it is barely immersed in the melt (i.e., the molten nutrient). To maintain a constant contact temperature between the melt and the entire circumference of the seed crystal, the rod is constantly rotated. As nutrient material attaches itself to the seed and crystalizes (a process that is assisted by the seed's attachment to the relatively cooler rod), the rod is slowly raised, pulling the growing crystal out of the melt. The growing tip is kept in contact with the melt until all the nutrient has been used. The rate of growth can be quite rapid, up to a rate of 4 in (100 mm) per hour. Very large crystals can be pulled, with diameters exceeding 2 in (50 mm) and lengths reaching 40 in (1 m) or more.
3 (Flux Growth) Flux is any material that when melted will dissolve another material that has a much higher melting point. Although temperatures in excess of 3,600° F (2,000° C) are needed to melt aluminum oxide, the material will dissolve in certain fluxes at a temperature as low as 1,470° F (800° C). Process temperatures above 2,200° F (1,200° C) are generally used because they produce higher-quality crystals. While dissolved in the flux, ruby molecules can travel freely and attach themselves to a growing crystal. Some manufacturers immerse seed crystals in the solution, and others simply allow the molecules to combine randomly and form an unplanned number of crystals. The temperature is maintained for a period of three to 12 months. Some manufacturers then pour off the still-molten flux to expose the ruby crystals. Other manufacturers cool the material slowly (4° F or 2° C per hour) and then extract the ruby crystals by breaking off the solidified flux or dissolving it in acid.
4(Hydrothermal Process) Powdered or crystalline nutrient is placed at one end of a pressure-resistant tube. A seed crystal is mounted on a wire frame near the other end of the tube. An appropriate water-based solution is placed in the tube, which is sealed shut. The tube is placed vertically in a furnace chamber, with the nutrient-containing end of the tube resting on a heating element. As the floor of the furnace is heated, the bottom end of the tube becomes hotter than the top (about 835° F or 445° C, compared to 770° F or 410° C); dissolved nutrient material migrates toward the seed and crystalizes on its relatively cooler surface. Pressure within the tube can range from 83,000-380,000 kPa (12,000-55,000 lb per sq in), depending on the amount of free space left in the tube when the solvent was inserted.
The tube used for the hydrothermal process can be made in any appropriate size, with a height-to-diameter ratio ranging from 8-16. In an example described in Synthetic Gem and Allied Crystal Manufacture, five seed crystals were placed in a 12 in (300 mm) long tube; each crystal grew at a rate of 0.006 in (0.15 mm) per day during the 30-day processing period.

Surface finishing

Whether it will be used as a gem or an industrial device, the ruby must be given a smooth, glossy finish after it has been cut or faceted to the desired shape. The following methods may be used.

5 (Polishing) The surface is rubbed with increasingly fine particles of an abrasive such as diamond powder. This traditional technique leaves only microscopic scratches and pits.
6 (Glossing) After initial polishing, the surface of the stone may be heated rapidly in a gas flame to melt any tiny projections. The surface is then allowed to cool, and the thin layer of molten material solidifies as a smooth surface. Treating ruby rods in this way nearly doubles the rod's tensile strength (resistance to a pulling force).

Comparing Synthetic To
Natural

Rubies, grown as rods for industrial use, are readily recognizable as synthetic because of their shape. Manmade stones that are cut as gems are not so easily identified. However, microscopic examination can reveal characteristic patterns of inclusions (foreign particles), bubbles, and striations (growth bands) that can distinguish between natural and synthetic stones, even revealing the location from which a natural stone came or the process by which a synthetic stone was made.