Like Charl just pointed out, if these had actually been altered by man to create the shapes then you would evidence of that. There would be grinding marks and peck mars on them and there aren't any on yours.
Take a look at these. I am no skilled artisan by any means, but yet I was able to create these three pieces out of Ohio Pipestone. It took some time and effort to do, but if I can do it then most others could too if they took the time to try. If a Native American wanted to create an Effigy of an object it would look just like that object and you wouldn''t have to twist it several directions to see the image. You can take one look at these and know without a doubt what the represent.