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How do chromium impurities give ruby its red color?

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A rough, translucent yellow crystal of elemental sulfur displayed against a solid black background. — crystal A crystal form of elemental sulfur. Sulfur has an atomic weight of about 32.06 atomic mass units (amu), which means that its molar mass is 32.06 grams.

Pure corundum (aluminum oxide) is naturally colorless because its electrons are paired, requiring high-energy ultraviolet light for excitation. The red color of ruby emerges when a small number of aluminum atoms—approximately 1 in 5,000—are replaced by chromium ions.123

These chromium ions possess unpaired electrons that create specific energy levels within the crystal's ligand field. When white light strikes the ruby, the chromium ions absorb photons in the violet and green-yellow parts of the spectrum, raising the system to higher energy states. The remaining light that passes through the crystal is dominated by the red portion of the spectrum, giving the gemstone its characteristic deep red hue.42

Additionally, when the chromium ions return to their ground state, they emit a portion of the absorbed energy as a bright red fluorescence, which further enhances the stone's vivid appearance.42