This is a relatively well developed example of what is called an odontolite, a mineralized tooth fossil. The mineral replacing the original tooth (apatite) is turquoise, and in this case with unusually good quality and fidelity to shape. The color is amazing. This is actually a valid turquoise pseudomorph after apatite, too! Such specimens are incredibly rare; and thought to be ice age fossils of some marmot-like rodent animal.