Which elements primarily cause sapphire’s blue color?

• A. Copper and nickel
• B. Iron and Titanium
• C. Chromium and vanadium
• D. Cobalt and manganese

Answer: (B)

Sapphire’s blue color comes from trace impurities in its crystal structure. In corundum (Al2O3), small amounts of iron and titanium substitute for aluminum ions in the lattice. These impurities create electronic interactions that absorb certain wavelengths of light, and the wavelengths that get absorbed shape the color we see. A common way to describe it is that iron and titanium together produce a charge-transfer effect, which preferentially absorbs red-yellow light, leaving blue to be transmitted to our eyes. The exact shade depends on how much of each impurity is present and their oxidation states.

Other elements can color sapphires in different ways or produce blue in rare cases (for example, chromium produces ruby, and cobalt can color sapphires blue under certain conditions), but the typical blue hue of sapphire is principally due to iron and titanium.