I am the fields rippling in the summer breeze, the depths of the forest and the ocean alike. I am the gleaming jewel of Erin, and the mark of some who fight over her today. I am the lesser of two brothers, mistaken for my twin in the light of the rising sun. I am the signal beckoning onward, drawing men into motion. I am the field of luck and folly, on which card and cube do play. I am the eyes that burn with desire for that which is not mine. I am the coat of the fourth horse.
Who am I?