The difference between a ruby and corundum is not a difference in fundamental mineral type, but rather a difference in chemical purity. Both ruby and sapphire are gem varieties of the mineral corundum, a substance that is colorless in its purest form. The distinction that separates the prized red ruby from the rest of the corundum family is the presence of a specific trace element that radically alters how the crystal interacts with light.
Corundum: The Shared Chemical Foundation
Corundum is a crystalline form of aluminum oxide, which is represented by the chemical formula Al2O3. This fundamental composition is shared by every stone in the corundum family, from the red ruby to every color of sapphire. The mineral structure is defined by its trigonal crystal system, where aluminum ions (Al3+) are precisely arranged within a dense packing of oxygen atoms.
This highly stable, repeating atomic arrangement is responsible for corundum’s exceptional hardness, making it the third hardest natural mineral after diamond and moissanite. In its chemically pure state, corundum is transparent and colorless, a variety sometimes called leucosapphire. The colors that define the gem varieties are a result of trace elements incorporated during the crystal’s growth.
The Specific Role of Chromium in Ruby
The vibrant red of a ruby is entirely dependent on the presence of the element chromium. During the formation of the crystal, small amounts of trivalent chromium ions (Cr3+) substitute for the aluminum ions (Al3+) within the fixed Al2O3 lattice.
This substitution occurs because the aluminum and chromium ions are similar in size, allowing the chromium to slip into the crystal structure without disrupting the overall framework. The concentration of chromium needed is remarkably small, often just a few hundredths of a percent.
The Cr3+ ions absorb light wavelengths in the yellow-green part of the spectrum. This selective absorption prevents yellow and green light from passing through the stone. The remaining light, which is primarily red and a small amount of blue, is transmitted to the eye, producing the characteristic red color of ruby. Furthermore, this chromium content often causes the stone to fluoresce under ultraviolet light, which can enhance the intensity and vividness of the red hue.
How Other Trace Elements Create Sapphires
While chromium creates the red color that defines ruby, other trace elements substituting into the same corundum structure are responsible for the wide array of colors seen in sapphires. The blue color most commonly associated with sapphire, for example, is not caused by a single element, but by the combined presence of iron (Fe) and titanium (Ti). These two elements engage in a phenomenon called intervalence charge transfer, where an electron is transferred between the iron and titanium ions.
This charge transfer mechanism absorbs the yellow and red wavelengths of light, allowing only the blue light to be transmitted to the viewer. Yellow and green sapphires, on the other hand, typically owe their color to the presence of iron alone. Pink sapphires are colored by trace amounts of chromium, just like ruby, but at concentrations too low to be classified as the deeper red of a ruby.
Geological Environments and Trace Element Availability
The existence of a ruby or a specific sapphire color is a direct consequence of the geological environment where the corundum formed. Corundum requires a formation environment that is rich in aluminum but notably poor in silicon. This mineral can form in both metamorphic rocks, such as marble, and in certain igneous rocks, like basalts.
Ruby is rarer than blue sapphire because its required trace element, chromium, is generally less abundant in the Earth’s crust than iron and titanium. High-quality rubies often form in marble deposits, which are naturally low in iron. This low iron content is important because iron can mute the red fluorescence caused by chromium, leading to a darker, less intense red. Conversely, many blue sapphires are found in basaltic rocks, which are much richer in iron and titanium, making blue corundum more common.