At http://geology.com/stories/13/ammolite/ … gems made from iradescent ammolite are quite striking – a confusion of colours. The colour and gem quality come from a thin iradescent shell material found in two species of extinct ammonite. You know, those nautilus fossils you find embedded in walls and buildings and which fall out of the Jurassic cliffs of Dorset. However, ammolite comes from a particular locality, a small arc along the St Mary River in SW Alberta in Canada.
The colour producing shell layer is thin and a backing material is necessary, such as shale or siderite. The deposit, we are told, goes back 75 million years ago on the uniformitarian time scale, to a time when the Rocky Mountains were in the process of being raised up. To the east of the mountains there was a broad body of water, it is claimed, and rain falling on the flanks of the mountains washed sediments into the sea way. These coalesced into a rock unit known as the Bearpaw Formation. The Bearpaw, however, is composed primarily of marine shells – and als contains some sandstone and layers of volcanic ash. We may note the sediments also contain fossils of bony fish, shellfish, sharks, sea turtles, and ammonites. Ammonites were buried in sediment and many of them became the nucleus of siderite concretions – in the Bearpaw. These ammonite fossiles, inside the concretions, sometimes have an outer shell layer of ammolite, and is minded economically on the St Mary River location.