What element primarily causes ruby’s red color?

• A. Iron
• B. Chromium
• C. Titanium
• D. Vanadium

Answer: (B)

Chromium is the element that gives ruby its distinctive red color. Rubies are the red variety of corundum, which is aluminum oxide. When Cr3+ ions substitute for some Al3+ in the crystal lattice, they introduce specific electronic transitions that absorb light in part of the spectrum—typically the green region. With those wavelengths absorbed, the stone reflects or transmits red light, producing that vivid ruby hue. The depth of the red depends on how much chromium is present: trace amounts yield lighter pinkish reds, while higher concentrations produce a deeper crimson. While other elements can affect color in other gems or in different doped combinations, the defining red color of ruby comes from chromium substituting into the aluminum oxide lattice.