The Treasure State
The official nickname for Montana is "The Treasure State" because of its rich mineral reserves. The mountains of Montana have yielded fortunes in gold and silver since the first substantial deposits were discovered in the mid 1800's (Montana's state motto is Oro y Plata - Spanish for "gold and silver "). Montana also has sapphires (the most valuable gemstone found in America).
Big Sky Country
Another nickname for Montana is "Big Sky Country," made popular by a Montana State Highway Department promotion in the 1960's (originating from a book by Alfred Bertram Guthrie Jr. titled Big Sky).