Ruby : The king of gemstones

 


Once scientists began to understand chemistry, they learned that ruby and sapphire are made from a special combination of aluminum and oxygen atoms that is described scientifically as aluminurn oxide (Al203) and usually referred to by ceramists simply as alumina. Chemists found out that ruby is red because it contains a tiny amount of chromium along with its aluminum oxide. There’s a bit of aluminum oxide in the Earth’s crust (called corundum), but we rarely see it as the gem-quality crystals of ruby or sapphire that we so admire. 

Our age-old dream of creating In the nineteenth century, researchers tried to change corundum into gems by heating it to a high temperature. They failed. Others tried chemical synthesis-building the gems from scratch. In 1837, the French scientist Marc Antoine Augustin Gaudin heated a chemical compound called ammonium alum (which turns into aluminum oxide when it’s heated) with a high-temperature torch. He actually did produce some small crystals of corundum, in much the same way that sugar crystals form along a string left hanging in a glass of sugar water as the water evaporates; but the crystals were not gem quality, not good enough to be used in jewelry. Many alchemists, scientists, and entrepreneurs tried diligently throughout the 1800s to synthesize gems, but no one was successful. Then, in 1902, Professor A.V.L. Verneuil of the Museum of Natural History in Paris sur- prised the world by unveiling synthetic rubies. 

The Verneuil method (or Verneuil process or Verneuil technique), also called flame fusion, was the first commercially successful method of manufacturing synthetic gemstones, developed in the late 1883  by the French chemist Auguste Verneuil. It is primarily used to produce the ruby, sapphire and padparadscha varieties of corundum, as well as the diamond simulants rutile and strontium titanate. 

One of the most crucial factors in successfully crystallising an artificial gemstone is obtaining highly pure starting material, with at least 99.9995% purity. In the case of manufacturing rubies, sapphires or padparadscha, this material is alumina. The presence of sodium impurities is especially undesirable, as it makes the crystal opaque. Depending on the desired colouration of the crystal, small quantities of various oxides are added, such as chromium oxide for a red ruby, or ferric oxide and titania for a blue sapphire.

A simplified diagram of the Verneuil process

This starting material is finely powdered, and placed in a container within a Verneuil furnace, with an opening at the bottom through which the powder can escape when the container is vibrated. While the powder is being released, Oxygen is supplied into the furnace, and travels with the powder down a narrow tube. This tube is located within a larger tube, into which Hydrogen is supplied. At the point where the narrow tube opens into the larger one, combustion occurs, with a flame of at least 2,000 °C (3,630 °F) at its core. As the powder passes through the flame, it melts into small droplets, which fall onto an earthen support rod placed below. The droplets gradually form a sinter cone on the rod, the tip of which is close enough to the core to remain liquid. It is at that tip that the seed crystal eventually forms. As more droplets fall onto the tip, a single crystal, called a boule, starts to form, and the support is slowly moved downward, allowing the base of the boule to crystallise, while its cap always remains liquid. The boule is formed in the shape of a tapered cylinder, with a diameter broadening away from the base and eventually remaining more or less constant. With a constant supply of powder and withdrawal of the support, very long cylindrical boules can be obtained. Once removed from the furnace and allowed to cool, the boule is split along its vertical axis to relieve internal pressure, otherwise the crystal will be prone to fracture when the stalk is broken due to a vertical parting plane.


Synthetic ruby boule produced by the Verneuil proces

Verneuil’s rubies attracted international attention. Within a few years, artificial gemstones and other products, including “jewel” bearings for watches and scientific instruments, were common. Nearly 440,000 pounds (about the weight of 73 three-ton elephants piled one on top of another) of such stones were produced in the world every year before the invention of the quartz watch dramatically reduced the need for jewel bearings. 

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