The Collaborative International Dictionary of English v.0.48:

Emerald \Em"er*ald\, a.
   Of a rich green color, like that of the emerald. "Emerald
   meadows." --Byron.
   [1913 Webster]

   Emerald fish (Zo["o]l.), a fish of the Gulf of Mexico
      (Gobionellus oceanicus), remarkable for the brilliant
      green and blue color of the base of the tongue; -- whence
      the name; -- called also esmeralda.

   Emerald green, a very durable pigment, of a vivid light
      green color, made from the arseniate of copper; green
      bice; Scheele's green; -- also used adjectively; as,
      emerald green crystals.

   Emerald Isle, a name given to Ireland on account of the
      brightness of its verdure.

   Emerald spodumene, or Lithia emerald. (Min.) See
      Hiddenite.

   Emerald nickel. (Min.) See Zaratite.
      [1913 Webster]