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Ammolite Mineral Facts:

Chemical Formula: CaCO₃
First, Ammolite is a fossil, not a mineral - therefore it has no exact  chemical formula as a true mineral does. The matter making up the iridescent material is the fossilized remains of a seagoing animal, and is made mostly of arragonite. As the remains of a living creature, there are many impurities within the minerals comprising the specimen. These fossils are valued for their spectacular array of iridescent colors. It is included in this list because of their interest to mineral  collectors. Specimens are also nearly always sealed or preserved with a clear coating to prevent degradation of the colorful but fragile shell material.

These beautiful specimens are the fossilized remains of Ammonites, an extinct group of marine invertebrate animals known as Cephalopods. Although they appear very similar to  the living shelled Nautilus species, these mollusks actually are more closely related to living octopuses, squid, and cuttlefish.

Colors: Various iridescent shades of red, green, blues, purple, yellows, orange, etc.

Hardness: 3.5 to 4
Soft for a gemstone.

Density: 2.95
The density is that of arragonite. 

Cleavage: One distinct cleavage on 010, one poor cleavage on 110, parallel to macropinacoid.  This is the same as arragonite, but in practice as the carbonate mineral is microscopic, no cleavages are ever seen.

Crystallography: Orthorhombic
No crystals are ever visible.

Luster:.Vitreous, transparent to translucent.

Optics: (Refractive Index):  a = 1.53; b = 1.68; y = 1.685

Composition, Structure and Associated Minerals:
The name ammonite, from which the scientific term is derived, was inspired by the spiral shape of their fossilized shells, which somewhat resemble tightly coiled rams' horns. Some fossil ammonite shells, such as many found in Madagascar and Alberta (Canada), display iridescence. These iridescent ammonites are often of gem quality (ammolite) when properly polished.

Identification and Diagnostics
Ammolite is not easily imitated nor are such imitations often seen; A few materials have a passing resemblance that may deceive those who are unfamiliar with the genuine article. These include: Abalone shell, gem labradorite (also known as spectrolite), an iridescent feldspar; and broad-flash black opal. None of these are very convincing substitutes, and the opal is actually of greater value than ammolite.  The visible structure of the ammolite is considerably different from the imitations, so telling them apart is not difficult.

Occurrence, Localities and Origins:
This material is valued as a fossil, but is more importantly used as a gem stone. Gem material has also marketed under the name "Korite". The beautiful gem layer is extremely thin, less than one millimeter thick. This is why the gem layer needs protection from exposure to the elements. Most specimens are infused with epoxy in a manner similar to some turquoise gems, and jewelry pieces are made as triplets with a durable cover and backing. Its history as a gem is relatively recent and is still growing as more people come to know of it. The ammonites that form gem ammolite inhabited a prehistoric, inland subtropical sea that bordered along the Rocky Mountains. This area is known today as the Cretaceous Interior Seaway. As the waters receded, the ammonites were buried by layers of clay sediments. This sediment preserved the elements of their shelled remains, preventing it from converting to calcite.

Significant deposits of gem-quality ammolite are found in the Bearpaw formation that extends from Alberta to Saskatchewan in Canada and south to into Montana in the USA. Most commercial mining operations have been conducted along the banks of the St. Mary River, in an area south of and between the town of Magrath and the city of Lethbridge.

Commercial extraction is mechanized but fairly basic: shallow open pits are dug in the shale with a backhoe and the excavated material is screened for its potential gem contents. The open pits are further examined by individuals, and the commercial production is supplemented by these individuals who sell their findings to the commercial ammonite producers. The ammolite deposits are stratified into several layers: the shallowest of these layers lies about 15 meters below the surface and extends to 30 meters down. The ammolite within this layer is covered by siderite concretions and is usually cracked. This is the most common material. Beginning twenty meters below the crushed material is a zone where the ammolite is usually compressed with a thin layer of pyrite rather than siderite concretions. This is the sheeted material that is of higher quality, but due to its depth is rarely mined.

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