MONTANA. THE SAGITTARIUS BIRTHSTONE COLOR IS REGARDED AS SYMBOLIC OF CURIOSIT Y, ENERGY AND SPIRIT OF ADVENTURE. Those born in December, or under the star sign of the archer, have smoky blue Montana as their birthstone color. Known for their energy and generosity, their curiosity and their need for freedom, Sagittarians are born adventurers. Montana is reminiscent of the calm, fathomless ocean and endless clear sky that show Sagittarian travelers the way and guide them to the shore, symbolizing reassurance and security in their journey through life.