Rubies and sapphires are varieties of corundum, a common mineral that can be transparent and used as a gemstone. Synthetic production of corundum began in 1837, with the Verneuil process introduced in 1903 for mass production of flawless rubies and sapphires. The process involves melting highly purified alumina powder using a blowtorch and accumulating droplets on a cylindrical crystal. The average commercial boule produced is 13mm in diameter, 25-50mm long, and weighs approximately 125 carats. It sells for between $1 and $200 per carat.
Rubies and sapphires are different varieties of the mineral corundum, also known as aluminum oxide. Aluminum oxide is extremely common, making up over 15% of the earth’s crust, but is usually impure, appearing as an opaque rock. When corundum is very pure, it is transparent and is considered a gemstone. Red corundums are called rubies, while all other colors (most frequently blue) are called sapphires. Corundum is prized in part because of its extreme hardness: the only naturally occurring mineral of greater hardness is diamond. A ruby can scratch just about anything except a diamond.
Synthetic production of ruby and other corundum began in 1837, when chemist Gaudin made the first synthetic rubies by fusing chromium (pigment) with alumina at high temperatures in an oxygen-containing environment. In 1847, Edelman synthesized white sapphire by fusing alumina in boric acid. In 1877 Frenic and Freil synthesized corundum crystals from which small stones could be cut. But it wasn’t until 1903 that Frimy and Auguste Verneuil introduced the Verneuil process, also called flame casting, for the mass production of rubies and sapphires. The Verneuil process allows for the creation of far larger and more flawless rubies than nature could produce.
The basic principle of the Verneuil process is to melt a highly purified alumina powder (>99.9995%) using a blowtorch at 2000°C (3600°F), slowly accumulating the droplets on a boule (cylindrical crystal). The average commercial boule produced by this process is 13 mm (0.5 inch) in diameter, 25 to 50 mm (1 to 2 inch) long, weighing approximately 125 carats (25 g). It will sell for between $1 US Dollar (USD) and $200 USD per carat. In order for the Verneuil process to take place smoothly, several conditions must be met: the flame temperature cannot be much higher than the minimum melting temperature, the contact area between the boule and its base must be as small as possible and always maintain the melted product in the same part of the blowtorch.
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