This is a Fossil Ammolite. This was found in Calgary, Canada. This piece measures approx. 40 x 29 mm: thickness is 10 mm: weight is 18 grams. 
 
Ammolite found primarily along the eastern slopes of the Rocky Mountains. It is made of fossilized shells of ammonites, which in turn are composed primarily of aragonite. Ammolite is also known as aapoak (Kainah for "small, crawling stone").