Which element is responsible for giving ruby its red color?

The red color of ruby is primarily due to the presence of chromium. When chromium ions replace some of the aluminum ions in the crystal structure of corundum (the mineral family to which ruby belongs), they absorb specific wavelengths of light and reflect others. This absorption of light gives rise to the characteristic red hue of rubies.

Chromium is crucial because it energetically allows the stone to absorb light in a way that occurs specifically with wavelengths associated with red colors. This is why chromium is the key element responsible for the vibrant red of rubies, distinguishing them from other gemstones and varieties of corundum, like sapphires, which can take on a range of colors based on different trace elements.