The colors of rubies and emeralds are so striking that they define shades of red and green—ruby red and emerald green. But have you ever wondered how they get those colors?

Both rubies and emeralds are minerals, which is a type of rock with a consistent chemical composition and a highly ordered structure at the atomic level.

Emeralds are mainly made up of the mineral beryl, which is made from the elements beryllium, aluminum, silicon and oxygen. Beryl’s crystal structure is more complicated than corundum’s because ...