- Morton gneiss, also known as rainbow gneiss, is an [Archean](/en.wikipedia.org/wiki/Archean/-age gneiss found in the Minnesota River Valley of southwestern Minnesota, United States. It is one of the oldest stones on Earth, at about 3.5 billion years old. Along with the nearby Montevideo Gneiss, it is the oldest intact continental crust rock in the United States. Its type locality is in Morton, Minnesota.