I believe what you have is the weight off a decoy.
Here are some exampleshttps://www.google.com/search?q=decoy+weights&biw=1280&bih=683&tbm=isch&tbo=u&source=univ&sa=X&ei=5OgtVPrsNKbksASujoLABw&sqi=2&ved=0CFYQsAQ
It does resemble the google examples only its not metal and the construction seems to perfect for stone....like it did come out of a mold....Could it be old, maybe 1800s or something like that?