Ammolite is the gem-quality, iridescent, fossilized shell of Placenticeras meeki and Placenticeras intercalar ammonites from the Upper Cretaceous marine shales of the Bearpaw Formation. Ammonites are a marine mollusc, member of cephalopod class, that became extinct approximately 65 million years ago.
In 1981, the World Jewellery Confederation gave ammolite official gemstone status, and in 2004 it was named Alberta’s provincial gemstone. Southern Alberta, mainly along the St. Mary River west of Magrath, appears to be the only region in the world where shells reaching gem quality are found. The mechanism producing gem-quality material is uncertain but is probably a combination of rapid burial, heavy pressures, and unique chemical reactions that preserve the iridescent shell structure. An ammonite shell of gem quality is rare; estimates are that less than 10% of shells found in the St. Mary River area are gem quality, and only 20% of these can be used for jewellery.
Because ammolite is soft and occurs in thin, delicate sheets, development of special processing techniques was essential for commercial application. Most material is impregnated with a synthetic resin to stabilize the flakey ammolite before cutting, and it is usually covered with a convex cap of spinel or other durable, transparent, synthetic material. The largest commercial producer of ammolite in Alberta, with about 90% of world production sells ammolite jewellery in more than 25 countries.
- Alberta Ammonite Shell Activity Map – Alberta Energy
- Alberta Ammonite Shell Activity Map, Lethbridge area – Alberta Energy