Might be salt or alkaline glazed crockery and early colonial like smokeythecat mentioned. The larger piece looks kind of like soapstone though, is it slick when it's wet? Hard to date them when you can't have them in hand but I would definitely keep them and look for more. I have found some crockery in my creek where I normally find Native American pottery and the local archeologist loved seeing them, they have a story and it's pretty cool. Nice finds!