HUGE Silver and Turquoise Belt Buckle

My father in law recently moved in with us and he found a box full of Native American jewelry and belt buckles. But he specifically asked me if I could find out how much this one is worth. It is 7 3/4" wide and 5" tall. The back is marked sterling. Can anyone help me value of this buckle, I’m sure I will have a lot more to share in the near future?