Published in England in: Gems & Jewellery: Section - Gems and Minerals.

Creating corundum.
The production of synthetic star rubies and sapphires

2014, March/Volume 23/No. 2, pp. 18-22

Fig. 1
The tour through the beautiful Swiss Alps to Monthey.

Martin Steinbach discusses the history of the production of synthetic rubies and sapphires, and visits Djeva, one of the largest producers of synthetic stones.

In many cases, synthetic gemstones are more beautiful, more perfect and can be bought at a much more favorable price than their natural counterparts. For quite a while I had the intention to pay a visit to the largest producer of synthetic stones, Djeva, based in Monthey, Switzerland. Its produce includes rubies, sapphires, spinels, cubic zirconia (Djevalite), synthetic retiles (TiO₂) and laser crystals. Above all, I wanted to have a look at the famous crystal-growing process of star rubies and star sapphires, as well as get to know the practical Verneuil process. Jules Verne wrote: “Look with all your eyes, look”, and this is what I wanted to do.

The modest beginnings of synthetic gemstones

Fig. 2
„Rue des SAPHIRS“, quite similar to „Aquamarinstraße“ in Idar-Oberstein.

For many, ruby is the most beautiful of the top three gemstones, and prior to the nineteenth century it was probably a dream of most chemists and alchemists) to produce it artificially. In 1837 these attempts became successful when the French chemist Marc Gaudin created the first microscopic crystals from alumina by melting two smaller rubies together. By 1877, the chemist Edmond Frémy had devised an effective method for the commercial manufacture of synthetic rubies with the assistance of his employee Auguste Verneuil. They used molten baths of alumina and yielded the first gem-quality synthetic stones, mainly small rubies, used as jewel bearings of watches. These rubies were called rubies scientifiques.

Fig. 3
The „godfather“ of nearly all synthetic gems: Professor Auguste Verneuil.

Auguste Verneuil (1856-1913) began work in 1886 on the production of synthetic rubies ans within six years had achieved exceptional results. From 1904 he began to publish in detail the results of his work on the flame fusion process, or the Verneuil method, the first commercially successful method of producing synthetic rubies, and later synthetic sapphires, which earned him worldwide fame.

The method is primarily used to produce rubies (in various shades of red) and sapphires (in all colors), including star stones (4, 5), as well as diamond stimulants, such as synthetic rutile and strontium titanate (Fabulite). Synthetic spinels of all colors have also been produced using this method since the mid-1920s.

Fig. 4
Synthetic translucent star sapphires, with artificially induced fingerprints.

Djeva and the flame fusion process

The creation of the synthetic ruby and sapphire boules require the following constituents:

• The starting material: alumina particles (aluminum oxide, Al₂O₃) with a very high purity of 99,0%, which melt at approximately 2,100°C (6)
• Small amounts of metal oxides to influence the color of the boules. This is dependent on the colour of corundum required; of red rubies chromium oxide is added, of blue sapphires titanium oxide and ferric oxide are used, and of pink sapphires chromium oxide and manganese are used. A yellow color is caused by nickel and magnesium oxides, green by cobalt and vanadium oxides, and the alexandrite effect is caused by vanadium oxide. A colorless sapphire (a possible diamond imitation) is pure aluminum oxide. In order to produce asterism, approximately 0.1 – 0.3% of TiO₂ plus the color-causing substance(s) are added to the starting material.
• Water, which, at Djeva, is now split by electrolysis into hydrogen (H₂) and oxygen (O₂), and stored in large tanks (7, 8).

Fig. 5
Synthetic star ruby with artificial cracks and a star looking like Neptun's trident.

In the upper part of the Verneuil furnace (10) a cylindrical receptacle containing the starting material is suspended from a spring mechanism. At the bottom of the receptacle is a sieve. At regular intervals a small electric hammer knocks onto the receptacle, causing a small amount of the powder to fall out, and oxygen is piped in through a tube. In the melting fumace the oxygen and the aluminum oxide combine with hydrogen, which is piped into the middle part of the Verneuil furnace by a second gas tube. The gas reacts with the oxygen and forms oxyhydrogen.

Fig. 6
This is the material for synthetic dreams, very pure: Al₂O₃, aluminium oxide.

In the heat of the oxyhydrogen flame, the powder that trickles down with every beat of the hammer (today fully automatic) melts to form a cloud of minute droplets. The fine ‘rain’ of these molten droplets drizzles down onto a piece of synthetic corundum of a former boule that now serves as a seed crystal and determines a given or specific crystallographic orientatin. On this basis, a single pear-shaped crystal (boule) grows drop by drop, layer by layer. Like a stalagmite, the crystal slowly grows upwards towards the oxyhydrogen flame. While this crystal grows very quickly with the Verneuil method (approx. 0.5-2 cm per hour) in contrast ot other methods for the production of synthetic gemstones, the boule is slowly moved downwards with a lowering device in order to keep the growing crystal in the same temperature range, which varies from about 1,900°C to 2,400°C. The growing corundum boule and oxyhydrogen flame are surrounded by a small fireclay furnace with a cylindrical piercing. The flame, the fusion process and the growth of the boules are observed and controlled simultaneously through a window in the furnace (11).

The size of these artificially-grown crystals (9) ranges from lengths of approximately 2.0-5.0 cm, normally with diameters of 1.2-1.5 cm, and according to Nassau (1980) even up to 9 cm. They attain weights of approximately 150-500 ct. Boules of 750 ct have been produced for which only two to four hours growth time was needed.

After the boule has cooled it is taken out of the furnace. With a small stroke of a hammer, or by breaking off the tip with a pair of pliers, it is freed from internal tensions. As a result, the synthetic crystal splits along its longitudinal axis. The optical axis runs along the parting plane, which is very important for the orientation of the cut.

Fig. 7
Big containers in ruby red and sapphire blue keeping all the material for a lot of synthetics.

Fig. 8
To our cocktail, Djeva adds H₂ and O₂, oxygen and hydrogen.

Until World War II, synthetic Verneuil corundum was produced only in France, Germany and Switzerland. In 1947 the American Linde Air Products Company started to produce large quantities of synthetic star rubies (Schlossmacher, 1969) and blue synthetic star sapphires, as well as various other colors (12). These synthetic stones were mostly opaque with ‘perfect’ colors and stars. The name ‘Linde Stars’ became a brand; typical of the stones was the ‘L’ engraved on the bottom of the cabochons (13).

Fig. 9
In the center you see the typical Verneuil apparatus, not much modified for more than 100 years.

In 1974, Linde’s production of synthetic stones was suddenly stopped, citing ‘overseas competitors’ as the problem. This is possibly to do with competition from the producer of synthetic star corundums Wiede’s Carbidwerk, based in Freyung, Germany. Productin began at Wiede’s Carbidwerk in the early 1950s independently of linde, and the company produced large amounts of synthetics with the Verneuil method that differed only in minor details from its American counterparts.

While the technical apparatus for the Verneuil flame fusion process has constantly been developed and refined over nearly a century, the basic principle of the method still remains the same. Today, Djeva is probably the most important producer of synthetic gemstones worldwide using this method. German companies in Idar-Oberstein who use synthetic stones obtain their raw material from Djeva.

Fig. 10
So many gas burners, here the gas flame and the growth of the boule behind the glass are controlled.

Fig. 11
Synthetic facetable ruby material as a typical boule.

Djeva offers synthetic corundum in 37 colors including 11 different shades of red and pink and six shades of blue (Nassau, 1980) (14). It is interesting to note that Djeva also produces the refractometer prisms for gemologists.

Stars made from discs

Fig. 12
An ad for a synthetic star sapphire ladies ring, made by Linde in 1949, with Bacon`s philosophy.

As mentioned previously, approximately 0.1-0.3% of titanium oxide is added to the starting material to produce stones with asterism. Part of the titanium oxide vaporizes during the crystallization of the corundum boule, while the remaining amount is integrated into the corundum lattice as Ti₂O₃ and forms a solid solution. After growth the raw, colored boules are transparent.

When the boules leave the furnace they are not removed by parting like the ‘normal’ corundum boules; instead this is done by annealing or tempering. The rutile needles make the raw boules opaque and the rutile network formed is the cause of the optical effect. These special ‘star boules’ attain diameters of up to approximately 22 mm. the opaque boules are cut into discs, which facilitates the (normally manual) polishing process. The thickness of these discs varies. Depending on the demand, these are supplied as entire boules or discs.

Fig. 13
The typical Linde „L“ on the back of their synthetic stars.

In 15 a star is made visible by adding a drop of acacia honey on the flat disc – a method similar to that used by experienced cutters with raw natural star stones. The ‘inclusions’ are not gas bubbles but honey bubbles – an absolutely new and unknown kind of inclusion in gemstones! It is possible to cut flat stones (16) with very intense synthtetic stars where the rays extend to the edge of the cabochon.

In Idar-Oberstein and the surrounding villages the flat corundum discs are still cut today into normal calibrated cabochons as well as interesting free-forms and fancy shapes (17, 18 and 19)

Fig. 14
Some of the famous 37 different colors of Djeva`s synthetic corundum production.

Fig. 15
An opaque, blue star sapphire boule with a “star” made of sweet honey.

Fig. 16
These discs in quite an ugly brown will show a beautiful star when cut.

Fig. 17
Violet synthetic star sapphire with a very sharp star – like the ones synthetic gems always show.

Fig. 18
Nice fancy star, produced by Djeva, cut in Idar-Oberstein in 2012.

Fig. 19
Beautiful “honey-color” star sapphire, made in Switzerland, cut in Idar-Oberstein.

All pictures copyright Martin P.Steinbach.

Acknowledgements:
My thanks go to Kurt Blaser for the fascinating tour de fabrique in Monthey and especially to Katia Djevahirdjian.

References
Nassau, K., 1980. Gems Made by Man
Chilton Book Co., adnor, PA, USA
Schlossmacher, K., 1969. Edelsteine und Perlen
E. Schweizerbart. Stuttgart Germany

About the author
Martin P. Steinbach has been a Fellow of the German Gemological Association (FGG) and an Asian Institute of Gemological Sciences Accredited Gemologist (AG) since 1982 and 1983 respectively. He is gem merchant specializing in Burmese jadeite and asteriated gems.