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Ammolite is a very rare and precious opal-like organic gemstone found only along the eastern slopes of the Rocky Mountains of the United States and Canada. It is made of the fossilized shells of ammonites, which in turn are composed primarily of aragonite, the same organic mineral that makes up pearls. It is one of the three organic gemstones, the other two being amber and pearl.
The chemistry of ammolite is variable, and aside from aragonite may include calcite, silica, pyrite, or other minerals. Its crystallography is amorphous. Its hardness is 4.5 - 5.5, quite soft for a gemstone, and its specific gravity is 2.60 - 2.85.
Ammolite is the official gemstone of Canada and is also known as aapoak (Blackfoot for "small, crawling stone"), gem ammonite, calcentine, and korite. It comes from the fossil shells of the Upper Cretaceous ammonites Placenticeras meeki, Placenticeras intercalare and Baculites compressus. Ammonites were cephalopods, or squid-like creatures, that thrived in tropical seas until becoming extinct along with the dinosaurs at the end of the Mesozoic era.
The ammonites that form ammolite inhabited a prehistoric, inland subtropical sea that bordered the Rocky Mountains. As the seas receded, the ammonites were buried and crushed by layers of sediment. Their shelled remains were impregnated by trace elements present in these sediments, the most common of these being iron, which accounts for the predominance of the green colors.
Ammolite is only found in the Bearpaw formation that extends from Alberta to Saskatchewan in Canada and south to Montana in the USA. The best grade of gem quality ammolite is along high energy river systems on the eastern slopes of the Rockies in southern Alberta. Most commercial mining operations have been conducted along the banks of the St. Mary River, south of Lethbridge.
Gemstone quality
The value of an ammolite gemstone is determined by the following grading criteria: a) number of colors, b) brightness of colors (iridescence), and c) the way the colors play (chromatic shift and rotational range).
Ammonite (the raw ammolite substance) shell is comprised primarily of aragonite with trace elements of aluminium, barium, chromium, copper, iron, magnesium, manganese, silicon, strontium, titanium and vanadium. Gradings are labelled, from best to least best, AA, A+, A, and A-. A large array of color is displayed in ammolite, including all the colors found in nature. Generally red/green is more common than blue or purple, but there are certain hues, like crimson or violet or gold, derived from any of the primary colors that are very rare and in high demand. Chromatic shift is how the colors vary with the angle of light striking it. Rotational range is how far the specimen can be turned while maintaining its play of color (the best rotate 360 degrees). The spectral other property of the mineral aragonite that allows us to see the incredible play of colors across it's surface is its iridescence. Unlike most other gems, whose colors comes from light refraction, the iridescent color of ammolite comes from interference with the light that rebounds from stacked layers of thin platelets in the aragonite.
Ammolite is considered the rarest organic gem material. It is soft and delicate, requiring special processing techniques known only to a few experts specializing in this commercial industry.
See also
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