Ruby is a variety of the mineral corundum, which is crystalline aluminum oxide (\(\text{Al}_2\text{O}_3\)). The vibrant red color that defines ruby is not inherent to the aluminum oxide itself, but comes from a trace impurity known as a dopant. This coloration is due to the presence of chromium ions (\(\text{Cr}^{3+}\)) substituting for some of the aluminum ions within the corundum crystal lattice. For a deep, rich red color to develop, the starting materials must be nearly pure aluminum oxide powder, typically with a purity of 99% or greater.
The process of creating synthetic ruby begins with mixing this high-purity alumina with a small, carefully measured amount of chromium oxide (\(\text{Cr}_2\text{O}_3\)). Standard laser-grade ruby, for instance, often contains chromium at a concentration as low as 0.05 atomic percent. This small addition of chromium oxide is enough to impart the characteristic red hue when the material is subjected to high temperatures and crystallizes.
Rapid Production: The Verneuil Process
The Verneuil process, also known as flame fusion, represents the oldest and most commercially successful method for producing synthetic corundum, dating back to the early 1900s. This technique is favored for its speed and relative simplicity, making it a cost-effective way to generate large quantities of material. The process begins by placing the finely powdered mixture of aluminum oxide and chromium oxide into a hopper at the top of a specialized furnace.
The powder sifts down through an inverted oxyhydrogen torch via a controlled mechanism. The intense heat of this flame, which reaches temperatures between 2,000 and 2,200 degrees Celsius, melts the powder into tiny droplets. These molten droplets then fall onto a ceramic pedestal or a tiny seed crystal, where they fuse and solidify layer by layer, forming a single-crystal mass known as a boule.
As the boule grows, the pedestal is slowly lowered to keep the top of the crystal within the flame’s consistent temperature zone. This rapid growth rate, which can be up to two centimeters per hour, allows for fast production but introduces structural features. The resulting boule often exhibits internal stresses due to the large temperature gradient during cooling, along with characteristic curved growth lines or striae and occasional microscopic gas bubbles. A careful splitting process is necessary to relieve the internal strain that develops.
High-Purity Synthesis: Flux and Czochralski Methods
While the Verneuil process is fast and economical, other methods are employed when the application demands higher crystal purity and fewer internal defects. The flux growth method mimics the slow geological formation of natural rubies by dissolving the aluminum and chromium oxides in a solvent, called a flux, which has a much lower melting point than corundum itself.
A common flux used is molybdenum trioxide (\(\text{MoO}_3\)) or a mixture of borates and fluorides. The raw materials and the flux are heated in a platinum crucible to form a liquid solution, which is then cooled very slowly. As the temperature gradually decreases, the ruby components precipitate out of the solvent and crystallize onto a seed crystal or spontaneously form crystals. This slow process, which can take several months, results in crystals with a significantly lower density of structural imperfections and excellent optical clarity compared to flame-fusion rubies.
The Czochralski method, also called the “pulling” method, offers a third approach, balancing the speed of Verneuil with the high quality of flux growth. This technique involves melting the raw materials in a crucible, typically made of iridium, which can withstand the approximately 2,050 degrees Celsius required to melt the alumina. A small seed crystal of ruby is lowered until it just touches the surface of the molten material.
The seed crystal is then slowly rotated and withdrawn from the melt. As the crystal is withdrawn, the molten material solidifies onto the seed, forming a large, cylindrical single crystal. The temperature must be precisely controlled to ensure continuous, flawless growth. This method produces large, extremely pure crystals with minimal inclusions and much lower internal strain than the Verneuil method, achieving growth rates of around 25 millimeters per hour.
Industrial and Scientific Uses of Synthetic Ruby
Synthetic rubies are produced not just for use in jewelry, but primarily because of their exceptional physical properties, which make them indispensable in various high-precision industries. The material’s extreme hardness, second only to diamond, and its resistance to chemical corrosion make it an ideal material for precision components in mechanical applications where durability and minimal friction are required.
The most common mechanical use is in jewel bearings, which are tiny components found in high-end watches and specialized measuring instruments. These bearings reduce friction and wear at rotational points, ensuring the longevity and accuracy of the device. Rubies are also used in high-precision nozzles for waterjet cutting equipment due to their ability to withstand high pressure and abrasive flow.
The optical properties of synthetic ruby are equally significant, particularly its use in the field of photonics. Historically, a synthetic ruby rod was the active medium used in the very first functioning solid-state laser, demonstrated in 1960. The chromium ions in the crystal lattice absorb energy and then emit intense, coherent red light at a specific wavelength. Today, synthetic ruby continues to be used in laser systems for medical, industrial, and defense applications, valued for its optical clarity and stability under high-power conditions.