Ammolite is an organic gemstone similar to opal found primarily along the eastern slopes of the Rocky Mountains of North America. It is formed by the fossilized shells of ammonites, which in turn are mainly composed of aragonite, the same mineral contained in nacre, with a microstructure inherited from the shell. It is one of the few biogenic gemstones; others include amber and pearl.
The chemical composition of ammolite is variable and, in addition to aragonite, it can include calcite, silica, pyrite, or other minerals. The shell itself can contain a number of trace elements, including: aluminum; barium; chrome; copper; iron; magnesium; manganese; strontium; titanium; and vanadium. Its crystallography is orthorhombic. Its hardness is 4.5 to 5.5 and its specific gravity is 2.60 to 2.85.
A play of iridescent color similar to opal is shown in the fine specimens, mainly in shades of green and red; however, all spectral colors are possible. The iridescence is due to the microstructure of aragonite: unlike most other gemstones, whose colors come from the absorption of light, the iridescent color of ammolite comes from interference with light bouncing off the stacked layers of thin platelets that make up aragonite.
Significant deposits of gem-quality ammolite are only found in the Bearpaw Formation that extends from Alberta to Saskatchewan in Canada and south to Montana in the United States.
The best grade of gem-quality ammolite is found along high-energy river systems on the eastern slopes of the Rocky Mountains in southern Alberta. Most commercial mining operations have been carried out along the banks of the St. Mary River, in an area to the south and between the town of Magrath and the town of Lethbridge.
Ammolite deposits are stratified in several layers:
The shallowest of these layers, called "Zone K", lies about 15 meters below the surface and extends 30 meters downward. The ammolite within this layer is covered by siderite concretions and is generally cracked; This is the shredded material. It is the most common ammolite and (generally speaking) the least valuable. Starting twenty meters below the crushing material is the "Blue Zone"; The ammolite in this area, which spans 65 meters, is usually compressed with a thin layer of pyrite rather than siderite concretions. This is the material of the blade; due to its depth, it is rarely mined. It is also much less fractured and is therefore the most valuable type of ammolite.
Credits: Geology In
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