Post it in the Native American artifacts forum ( I say that bc it looks in the first picture as though there's a slim but possible chance it was chipped on by a Native American at some point.) Title your post as "Native American or natural?" And you will get several replies. In the description area, ask if anyone can tell you what type of material it is. There's some really sharp minds in that forum who will likely tell you that it's not Native American but it is an XYZ mineral. Good luck!
Ps-- very important to include what state & what part of the state it came from -- north south east or west.