The ruby is a highly prized gemstone, recognized for its vibrant red color and exceptional durability. While important in fine jewelry, its unique properties also made it valuable in scientific applications, such as the development of the first successful laser. Understanding the ruby requires examining the fundamental atomic arrangement that gives it both its distinctive hue and remarkable strength.
The Core Chemical Structure
The material that forms a ruby is the mineral corundum, which is crystalline aluminum oxide (Al₂O₃). This means two aluminum atoms are bonded with three oxygen atoms in its fundamental structure. Corundum is the most stable form of aluminum oxide, and pure corundum is naturally colorless.
The atoms within the ruby are arranged in a dense, highly ordered hexagonal crystal lattice. This compact atomic arrangement is responsible for the ruby’s extraordinary hardness. Corundum ranks 9 on the Mohs Hardness Scale, surpassed only by diamond.
This structural rigidity provides resistance to scratching and chemical alteration. The strength of the bonds between the aluminum and oxygen ions results in a very high melting point, contributing to the mineral’s stability. This underlying chemical framework provides the foundation for the gemstone, while trace elements introduce the characteristic color.
The Source of the Red Color
The brilliant red color that defines a ruby results from a specific crystal impurity, not the corundum structure itself. The vibrant hue comes from the trace element chromium (Cr). Chromium atoms substitute for a small number of the aluminum atoms within the Al₂O₃ crystal lattice.
Chromium ions alter the energy levels of the surrounding oxygen ions. This substitution causes the crystal to absorb specific wavelengths of light, primarily in the yellow-green and violet regions of the visible spectrum. The light that is not absorbed is transmitted, resulting in the perception of red.
This process also leads to fluorescence. The absorbed high-energy light excites the chromium ions, and as they return to a lower energy state, they re-emit light in the red part of the spectrum. This emitted red light combines with the transmitted red light, giving fine rubies their intense, glowing appearance.
Relationship to Sapphire
Ruby and sapphire are chemically the same material, both being varieties of corundum. They share the identical core Al₂O₃ structure and the hardness of 9 on the Mohs scale. The distinction between them is purely a matter of color, determined by the specific trace elements incorporated into the crystal.
Corundum that is red, due to the presence of chromium, is classified exclusively as ruby. All other color varieties of gem-quality corundum, including blue, yellow, pink, and green, are categorized as sapphire. Blue sapphire, for instance, gets its color from a combination of iron and titanium impurities.
The presence of different metal oxide impurities introduces various color agents into the corundum structure. Pink corundum is often considered a pink sapphire, though those with a deep saturation of chromium may sometimes be classified as ruby depending on the gemological standard used.
Natural and Synthetic Production
Natural rubies form deep within the Earth’s crust under specific geological conditions involving high heat and pressure. The formation process typically occurs in rocks rich in aluminum but low in silica, such as certain marbles or basalts. Finding the right mix of aluminum oxide and a chromium source is a rare occurrence, contributing to the high value of natural stones.
In a laboratory setting, rubies can be synthesized with the exact same chemical composition and crystal structure as their natural counterparts.
Verneuil Process (Flame Fusion)
One common industrial method is the Verneuil process, also known as flame fusion. This process melts aluminum oxide powder mixed with chromium oxide in an oxyhydrogen flame. The molten material then crystallizes rapidly into a large, cylindrical crystal mass called a boule.
Flux Growth
Another method is flux growth, which involves dissolving the raw materials in a molten solvent, or flux, at high temperatures. As the solution slowly cools, rubies crystallize over a period of many months. Synthetic rubies from the Verneuil process are often identifiable by curved growth lines, while flux-grown rubies can contain flux inclusions that mimic natural imperfections.