Binghamite (also called silkstone and cuyunite) is a type of agate stone found only on the Cuyuna iron range (near Crosby) in Crow Wing County, Minnesota. The formation of the stone occurs near deposits of iron ore.
Mining in the area uncovered deposits of the stone, but since mining operations were discontinued in the area many years ago the stone has become fairly rare and is sought after by lapidarists and gem collectors. The stone has areas that are highly chatoyant similar to pietersite or tiger's eye. It is a variety of quartz with fibers of goethite or hematite usually in colors of red, gold, and black. The highest quality binghamite compares with top pietersite for color and chatoyancy. When the stone's pattern is arranged in parallel bands it is called silkstone. Binghamite shows crystal growth in all directions. It is named after William Bingham, a lapidarist, who discovered it in 1936.