"Dig for clear, amethyst and citrine crystals. There is a $5 per car usage fee. "  This is from http://www.rockpicks.net/states/montana.htm

More information can be found at other websites like http://goldwest.visitmt.com/listings/11967.HTM or http://www.gorp.com/parks-guide/travel-ta-beaverhead-deerlodge-national-forest-montana-sidwcmdev_066349.html