Corundum is a crystalline form of aluminum oxide (Al₂O₃) known for its striking range of colors. It is extremely hard, ranking 9 on the Mohs scale, second only to diamond, making it valuable for industrial and gemological uses. The mineral hosts a spectrum of hues, giving rise to the well-known gemstones ruby and sapphire. Despite this, pure corundum is a simple oxide compound that is fundamentally colorless.
The Pure State: Corundum’s Chemical Identity
Pure corundum, sometimes called leucosapphire, is transparent and colorless. This lack of color results directly from its chemical composition and highly organized, trigonal crystal structure. The lattice is built from aluminum ions (Al³⁺) within a dense packing of oxygen ions (O²⁻).
In this ideal arrangement of Al₂O₃, there are no available electrons to interact with visible light energy. The energy required to excite the electrons in the ions exceeds the energy contained in the visible light spectrum. Consequently, all wavelengths of visible light pass through the material without being absorbed, resulting in a transparent appearance. The inherent hardness and stability of corundum are also attributed to this tightly bound, ordered crystal structure.
How Trace Elements Create Color
Corundum’s vibrant colors emerge when minute amounts of foreign elements, known as chromophores, are incorporated into the crystal lattice during formation. These impurities, often present in concentrations of just a few parts per million, substitute for the aluminum ions in the structure. Substituting an aluminum ion with a transition metal ion, such as chromium or iron, introduces new energy levels not present in the pure crystal.
These new energy levels allow the chromophore’s electrons to absorb specific wavelengths of light from the visible spectrum. The perceived color is the complementary color that is transmitted through the stone, not the color that was absorbed. For instance, if a chromophore absorbs yellow and green light, the remaining light transmitted appears red.
The Spectrum of Corundum Colors
The specific color created depends entirely on the identity and concentration of the trace element, or the interaction between multiple elements.
Ruby (Red Corundum)
The intense red color of ruby is caused by chromium (Cr³⁺) ions substituting for aluminum. A low concentration of chromium produces pink, which deepens into the distinctive red of a ruby as the concentration increases.
Sapphire (Blue and Fancy Colors)
The classic blue color of sapphire is caused by the interaction between iron (Fe²⁺) and titanium (Ti⁴⁺) ions. This interaction, known as intervalence charge transfer, strongly absorbs red and yellow light, allowing blue to be transmitted. Iron (Fe³⁺) alone is primarily responsible for yellow and some green hues.
The vast range of other colors are collectively known as fancy sapphires. Purple sapphires are often colored by a mix of chromium and the iron/titanium pair. Vanadium (V³⁺) can induce a purple or grayish-blue color, and in higher concentrations, it can cause the rare color-change phenomenon. Even minute variations in the valence state of these ions can shift the transmitted color, demonstrating the hypersensitivity of corundum’s color to its trace element chemistry.
Methods of Color Enhancement
Since color is important to corundum’s value, human intervention is commonly used to alter or improve the natural color.
High-Temperature Heat Treatment
The most widespread treatment is high-temperature heat treatment, involving heating the stone to temperatures often exceeding 1500°C. This process dissolves tiny crystalline inclusions, such as rutile needles, releasing constituent elements back into the corundum lattice. Reincorporating these elements can intensify or change the color, such as enhancing blue by freeing up titanium to pair with iron, or improving clarity by dissolving inclusions.
Diffusion Treatment
Another technique is diffusion treatment, where corundum is heated with chemical powders rich in chromophores like iron or titanium. The color-causing elements penetrate the surface of the stone, creating a thin layer of enhanced color. Beryllium diffusion, a variation of this method, can produce vibrant orange and yellow colors by allowing the small beryllium atom to penetrate deeply into the crystal structure.