Alexandrite is a unique type of gem that is a different color depending on the light by which it is seen. Often, it is described as being “emerald by day, and ruby by night.” That is because the gem is a bluish green in daylight or fluorescent light, but incandescent light or candlelight makes the gem a purplish red color.

The color change of alexandrite is the most important quality factor for the gem. However, clarity is also important, and most fine gems are eye clean. Alexandrites usually have few inclusions, but the rare parallel needle-like inclusions that can be present create a cat’s-eye phenomenon, which increase the value.

Along with the pearl and moonstone, alexandrite is the birthstone for the month of June and is also the gem for a 55th wedding anniversary.

The gem was first discovered in 1830 in Russia’s Ural Mountains, and was named after Csar Alexander II. While the Ural Mountains remain a source of alexandrite, it can also be found in Sri Lanka, Madagascar, Tanzania, and Brazil, which only recently became a new source in the last 1980s and early 1990s.

There are a number of famous alexandrites, including the Whitney Alexandrite, which is from a mine in Brazil and is in the collection of the Smithsonian Institution National Museum of Natural History. Another alexandrite that is part of the Smithsonian collection is a 66-carat gem from Sri Lanka.