This is a stone tool used for pounding hides. The discolorization on the flat surface is caused by years of blood being pounded into it.
Pretty sure it is Pawnee.
hey dano i find that the handfull of indian tools i have if you move them around in your hand you can feel the wear impressions to were the user/users had placed them for the proper use wear patterns etc.its kind of spooky thinking that there was a person who used this 5-10,000 yeras ago really a cool feeling,kiind of funny they will just fit into your hand with out really trying to hard.