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Article: Ammolite


Properties and Origin

  • Variable chemical composition, including aragonite, calcite, silica, pyrite, and other minerals
  • Orthorhombic crystal structure
  • Hardness of 3.5-4.5 on the Mohs scale
  • Specific gravity of 2.60-2.85g/cm³
  • Fluoresces a mustard yellow under ultraviolet light
  • Ammolite comes from the fossil shells of ammonites, specifically Placenticeras meeki, Placenticeras intercalare, and Baculites compressus
  • Ammonites thrived in a prehistoric subtropical sea that bordered the Rocky Mountains, known as the Cretaceous or Western Interior Seaway
  • Ammonites sank to the bottom of the sea and were buried by layers of mud, eventually becoming shale
  • Gem-quality ammonites are found within siderite concretions
  • Sediments preserved the aragonite of the shells, preventing it from converting to calcite

Occurrence and Extraction

  • Significant deposits of gem-quality ammolite are found in the Bearpaw Formation, extending from Alberta to Saskatchewan in Canada and south to Montana in the USA
  • Small deposits have been found as far south as Central Utah
  • Best grade of gem-quality ammolite is found along high energy river systems on the eastern slopes of the Rockies in southern Alberta
  • Most commercial mining operations are conducted along the banks of the St. Mary River, between the town of Magrath and the city of Lethbridge
  • Roughly half of all ammolite deposits are contained within the Kainah reserve
  • Commercial extraction is mechanized, involving shallow open pits dug with an excavator
  • Excavated material is screened for potential gem contents
  • Pits are further examined by hand
  • Approximately 50% of mined ammolite is suitable for jewelry
  • Korite, the largest miner of ammolite, produces over 90% of the world's supply

Other Names and Imitations

  • Ammolite is also known as aapoak (Kainah for small, crawling stone), gem ammonite, calcentine, and korite
  • The trade name 'korite' was given to the gemstone by the Alberta-based mining company Korite
  • Marcel Charbonneau and Mike Berisoff were the first to create commercial doublets of the gem in 1967
  • They formed Ammolite Minerals Ltd.
  • In 1981, ammolite was given official gemstone status by the World Jewellery Confederation (CIBJO)
  • Labradorite and broad-flash black opal have a passing resemblance to ammolite but are not convincing substitutes
  • Slocum stone, a glass-based imitation of opal, is even less convincing with tinsel-like patches of play of color
  • Ammolite can be grouped with shell-based marbles like lumachella, but the iridescence of lumachella is not as brilliant
  • Ammolite is often used as an imitation of black opal, which is actually of greater value

Gemstone Quality and Value Factors

  • The quality of gem ammolite is graded using a letter system (AA, A+, A, A-) but is not standardized
  • Primary colors in ammolite include red, green, blue, and purple, with red and green being more common
  • The rarest and most valuable ammolite grades have three or more primary colors or 1-2 bright and even colors
  • Chromatic shift refers to how the colors vary with the angle of viewing, while rotational range is how far the colors can be turned
  • The brightness of colors and iridescence depend on the preservation of the nacreous shell and the fine layers of aragonite
  • The number of primary colors and the presence of rare hues like crimson or violet affect the value of ammolite
  • Higher-grade ammolite shows a prismatic variation in chromatic shift, while lower grades have little variation
  • The brightness of colors and iridescence depend on the preservation of the nacreous shell and the quality of polish
  • Thicker ammolite layers, without visible matrix or foreign minerals, are more valuable
  • Calibrated stones and minimal treatment increase the value of ammolite gems

Use in Jewelry and Market

  • Ammolite is best suited for use in pendants, earrings, and brooches due to its fragility
  • It gained interest in Western society in the 1970s and entered the market in 1969
  • The Blackfeet tribe believes ammolite, known as 'iniskim,' possesses amuletic powers
  • Practitioners of feng shui promote ammolite as a stone that enhances well-being and detoxifies the body
  • Ammolite is usually fashioned into freeform cabochons and mounted in gold, often with diamond accents
  • Japan is the largest market for ammolite
  • Ammolite is used as an imitation of black opal, which is becoming scarce
  • Canada is another market for ammolite, used by artisans and fine jewelry producers
  • Ammolite is crafted in the Southwest United States by Native American craftspeople
  • It is popular among tourists visiting Banff National Park and Jasper National Park

Ammolite Data Sources

Reference URL
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