On natural stone there's no need to do miters. Just overlap the corners and leave the same size grout joint as the field on the abutting stone.
Use a diamond pad or sandpaper to relieve the sharp edge on the top of the face stone.

http://img337.imageshack.us/img337/6277/dsc1253edgedetail.jpg

Looks like you have what is called a "quirk miter". It's one method for making corners with stone or tile. If your corner isn't perfectly plumb it can turn out badly. It is also difficult to miter each tile with the same amount of "reveal" on the end. Better to do what RC says.