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How does chromium impurity give ruby its red color?

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A color-coded periodic table of the elements displays each element's symbol and atomic number, with categories such as alkali metals, noble gases, halogens, transition metals, and actinoid elements distinguished by different background colors. — transition metal Modern version of the periodic table of the elements (printable).

Ruby is a variety of the mineral corundum (aluminum oxide) that derives its characteristic red color from the presence of chromium as an impurity. While pure corundum is colorless, the replacement of a small number of aluminum atoms by chromium atoms—typically about 1 in 5,000—alters the crystal's optical properties.123

This color change is explained by ligand field theory. In the corundum structure, each aluminum ion is surrounded by six oxygen atoms in an octahedral arrangement. When chromium ions are introduced, their unpaired electrons interact with this surrounding electric field, creating specific energy levels.3

When white light passes through the ruby, the chromium ions absorb photons in the violet and green-yellow regions of the spectrum to excite these electrons to higher energy states. Because these specific wavelengths are absorbed, the light that is transmitted consists primarily of red and some blue, resulting in the deep red color of the gemstone. Additionally, the chromium ions can emit absorbed energy as a bright red fluorescence when the stone is exposed to ultraviolet radiation.41