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fossilized shell - ammon's stone
A lot of fossilized shell used in jewelry these days comes from Madagascar. However a variety of black ammonites is found in as unlikely a place (one might think) as the windswept Tibet plateau -- an area once covered by the ancient Tethys Sea.
Expanding its shell by adding larger chambers in a circular motion, the ammonite used its shell to control the depth at which it floated in the ocean.
The interior shell of the ammonite was very similar to the nautilus of today. Like the nautilus, ammonites were also related to cuttlefish and squid.
Used as index fossils (to indicate the age of a stratum), ammonites are considered a vital group for the study of the evolution of life.
Ammonite fossils as small as 3 mm (approx. 1/8") and as large as 3 meters (10') have been found.
The last living representative of ammonites is the nautilus which still lives in the Indian Ocean.
Neighbors remember the remarkable naturalist and scientist Rachel Carson as a child, telling a story that her romance with the ocean began one day when she found a large fossilized shell in the rocky outcroppings on the family's hillside property.
 ammonites and ammolites
One very popular shell used in jewelry is the long extinct ammonite, an ancient cephalopod, a prehistoric nautilus related to the existing nautilus, squid, and octopus families.

fossilized ammonite or ammolite

Among the very first life forms on earth, ammonites were very common in the oceans of the Paleozoic and Mesozoic eras, 400 to 65 million years ago, evolving in the Devonian Period. They became extinct at the end of Cretaceous Period of the Mesozoic Era.
Several hundred species of ammonites can be found around the world where they lived in the prehistoric oceans of 500 - 65 million years ago. A hard shell exterior once covered the soft insides of this creature which grew to be as large as 6 feet in diameter.

saharan ammonite
photo: paleodirect

One of the most famous fossil shell sites is the juxtaposition of the Atlas Mountains and the Sahara desert in Morocco. Ammonites there date back to the Devonian period -- the "Age of Fishes," roughly 355 - 410 million years ago, when the Saharan area was covered by a vast ocean. As a result of the conjuncture of receding waters, and natural disasters, this prehistoric ocean became what is now known as the Sahara desert, one of the world's richest formations for all sorts of amazing fossils.

burgess shale; canadian rockies
photo: burgess shale

Another site is the Burgess Shale, in the Canadian Rockies of British Columbia, discovered early in the twentieth century. Whereas most fossilization preserves hard tissues like bones, teeth, and shells, the geologic forces that formed the Burgess Shale also preserved soft tissues. The shale's fossil animals and plants are from the Cambrian era, long before the dinosaurs during which a remarkable array of living things came into being.
In fossilized form ammonite is known as ammolite (also referred to as Buffalo Stone, calcentine, or korite). Sometimes used as a gemstone, it is a gray, iridescent stone with flashes of blue, green, purple, red, or yellow. Like opals, the color changes as the stone is viewed from different angles. It is only found in southern Alberta, Canada.