This is a very high quality 10.8" iridescent Placenticeras ammolite from the Bearpaw Shale formation in Alberta, Canada.
Placenticeras is a genus of ammonites from the Upper Cretaceous. This specimen is at least 66 million years old.
Ammolite is an opal-like organic gemstone. The specimen shines mutliple hues of green, yellow, and red. It is made of the fossilized shells of ammonites.
Supplied with its metal base.