Ammolite is an opal-like organic gemstone found primarily along the eastern slopes of the Rocky Mountains of North America. It is a thin iridescent aragonite shell material that is found on two species of extinct ammonite fossils; the same mineral contained in nacre. Ammolite is one of few biogenic gemstones; others include amber and pearl, mother-of-pearl, coral, gagate (jet).
Ammolite is also known as AAPOAK (the Kainai Nation word for "small, crawling stone"), "ammonite shell", "gem ammonite", "calcentine", and "korite". The latter is a trade name given to the gemstone by the Alberta-based Canada mining company Korite International, the first and largest commercial producer of ammolite.
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 USA. However, small deposits have been found as far south as Central Utah which also contains gem-quality ammolite. The best grade of gem quality ammolite is along high energy river systems on the eastern slopes of the Rockies in southern Alberta.
The grade and therefore the value of an ammolite gemstone is determined by the following criteria: the number of primary colors (red and green are far more common than blue or purple), the way the colors "play" (how the colors vary with the angle of viewing and the angle of light striking the gemstone), brightness of colors (iridescence).
The thickness of the ammolite layer is also an important factor: after polishing, the ammolite is only 0.1-0.3 millimeters thick. Ammolite is usually fashioned into freeform cabochons and mounted in gold, with diamonds as accents. Due to its delicacy, ammolite is best reserved for use in pendants, earrings, and brooches.
