Ruby, Sapphire, and Emerald

How are ruby, sapphire, and emerald crystals related? How do they produce their colors?

Answer

How are ruby, sapphire, and emerald crystals related? Ruby and sapphire are color variations of the same mineral, corundum. Rubies contain a small amount of chromium. Pure corundum is a colorless, trigonal crystal that occurs in a wide variety of colors due to infiltrations of other elements. All color variations of corundum, with the exception of ruby, are called sapphires.

Rubies are red variations of the mineral corundum, a crystalline form of aluminum oxide and one of the most durable minerals that exists. Only diamonds are harder. Rubies’ rich, red colors arise from the substitution of a small number of aluminum atoms by chromium atoms. When exposed to high temperatures, rubies turn green, but they regain their original color after cooling. Some rubies phosphoresce with a vivid red glow when illuminated by ultraviolet light. Sapphire is aluminum oxide with trace impurities of iron and titanium atoms, which are responsible for the deep blue color shades most people associate with sapphire. Several other colors of corundum, such as yellow, reddish-orange, and violet, also are classified as sapphire. Synthetic sapphires have been produced commercially since 1902 and are used for scratch-resistant watch crystals, optical scanners, and in applications where physical strength and transparency to ultraviolet irradiation are important.

Emeralds are quite different from rubies and sapphires in that emeralds are the green form of beryl, colored by the presence of chromium or vanadium. The crystal structure of beryl emeralds is hexagonal (six-sided), with a hardness slightly higher than quartz but considerably less than diamond. Emeralds are notorious for containing flaws, and flawless stones are rare and greatly valued.

The colors in these gemstones are produced by characteristic downward atomic transitions involving F-centers (from the German word farbe, meaning color) at the chromium or other atoms. In a simple view of an F-center, the ambient light excites one electron in the chromium atom, for example, so the atom can be treated analogous to the hydrogen atom, with a large average radius for the location of the electron away from the chromium nucleus. The electron transition back to a lower energy state causes the emission of a photon in the visible.