Emerald is a green gemstone. It's also the name of this particular shade of green. On St. Patrick's Day, you'll see the color emerald everywhere — in cupcake icing and maybe even in your beer.
Green things are called emerald because of the emerald itself, which is a green, transparent form of beryl and a precious gemstone. Gemstones are found in many forms of jewelry, and they're especially loved in Ireland, which is also known as the Emerald Isle. Like many gemstone names, emerald has a long pedigree in many languages: most immediately French, Latin, and Greek.