Ammolite


Ammolite is an organic gemstone found primarily along the eastern slopes of the Rocky Mountains of North America. It is commonly unearthed by natural erosion or through the process of various mining practices, within the perimeter of an ancient sea bed called the Western Interior Seaway. It is made of the fossilized shells of ammonites, which in turn are composed primarily of aragonite, the same mineral contained in nacre, with a microstructure inherited from the shell. It is one of few biogenic gemstones; others include amber and pearl.
In 1981, ammolite was given official gemstone status by the World Jewellery Confederation, the same year commercial mining of ammolite began. It was designated the official gemstone of Lethbridge, Alberta, Canada in 2007, and was subsequently designated as Alberta's official gemstone in April 2022.
Ammolite is also known as aapoak, gem ammonite, calcentine, and korite. The latter is a trade name given to the gemstone by the Alberta-based mining company Korite. Marcel Charbonneau and his business partner Mike Berisoff were the first to create commercial doublets of the gem in 1967. They went on to form Ammolite Minerals Ltd.

Properties

The chemical composition of ammolite is variable, and aside from aragonite may include calcite, silica, pyrite, or other minerals. The shell itself may contain a number of trace elements, including: Aluminium, barium, chromium, copper, iron, magnesium, manganese, strontium, titanium, and vanadium.
An iridescent opal-like play of color is shown in fine specimens, mostly in shades of green and red; all the spectral colors are possible, however. The iridescence is due to the microstructure of the aragonite: Unlike most other gems, whose colors come from light absorption, the iridescent color of ammolite comes from interference with the light that rebounds from stacked layers of thin platelets that make up the aragonite. The thicker the layers, the more reds and greens are produced; the thinner the layers, the more blues and violets predominate. Reds and greens are the most commonly seen colors, owing to the greater fragility of the finer layers responsible for the blues. When freshly quarried, these colors are not especially dramatic; the material requires polishing and possibly other [|treatments] in order to reveal the colors' full potential.
The ammolite itself is actually a very thin sheet, in thickness. Rarely is ammolite without its matrix, which is typically a grey to brown shale, chalky clay, or limestone. So-called "frost shattering" is common; exposed to the elements and compressed by sediments, the thin ammolite tends to crack and flake; prolonged exposure to sunlight can also lead to bleaching. The cracking results in a tessellated appearance, sometimes described as a "dragon skin" or "stained glass window" pattern. Ammolite mined from deeper deposits may be entirely smooth or with a rippled surface.
Occasionally a complete ammonite shell is recovered with its structure well-preserved: fine, convoluted lines delineate the shell chambers, and the overall shape is suggestive of a nautilus, which is a different cephalopod type, species of which have survived to the present day. While these shells may be as large as 90 cm in diameter, the iridescent ammonites are typically much smaller. Most fossilized shells have had their aragonite pseudomorphously replaced by calcite or pyrite, making the presence of ammolite particularly uncommon.

Origin

Ammolite comes from the fossil shells of the Upper Cretaceous disk-shaped ammonites Placenticeras meeki and Placenticeras intercalare, and the cylindrical baculite, Baculites compressus. Ammonites were cephalopods, 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—this area is known today as the Cretaceous or Western Interior Seaway. As the ammonites died, they sank to the bottom and were buried by layers of bentonitic mud that eventually became shale. Many gem-quality ammonites are found within siderite concretions. These sediments preserved the aragonite of the shells, preventing it from converting to calcite.

Occurrence

Significant deposits of gem-quality ammolite have only been 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. 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. Roughly half of all ammolite deposits are contained within the Kainah reserve, and its inhabitants play a major role in ammolite mining.
Since its founding in 1979, Korite has operated primarily within the reservation. The company had an agreement with the Kainah tribe, with Korite paying the tribe royalties based on how much land the company has mined. This agreement has expired. It prohibited the Blood Tribe members from surface mining along the banks and cliffs of the St. Mary River. There were about 35 licensed Blood surface miners in 2018. The surface miners are self employed mining in all kinds of weather. Some miners also restore the fossils they find or resell their finds to other fabricators.

Extraction

Commercial extraction is mechanized but fairly basic: shallow open pits are dug with an excavator and the excavated material is screened for its potential gem contents. The pits are further examined by hand, and commercial production is supplemented by individuals who sell their surface-picked findings to Korite and several other producers. Approximately 50% of the ammolite mined is suitable for jewelry. Korite, the largest miner of ammolite, produces over 90% of the world's supply.
The ammolite deposits are stratified into several layers: the shallowest of these layers, named the "K zone", lies some 15 meters below the surface and extends 30 meters down. The ammolite within this layer is covered by siderite concretions and is usually cracked — this is the crush material. It is the most common and the least valuable form of ammolite. Beginning twenty meters below the crush material is the "blue zone"; ammolite from this zone, which extends 65 meters, is usually compressed with a thin layer of pyrite rather than siderite concretions. This is the sheet material; due to its depth it is rarely mined. It is also much less fractured, and therefore a more valuable form of ammolite.
, Korite has mined over 100 acres of ammolite deposits. The company employs over 280 people and accounts for approximately 90 percent of world gem ammolite production. Prospectors who wish to mine ammolite deposits on Crown land must apply to the Alberta Department of Energy for a lease. These leases are not regularly offered; as of 2004, there was a CAD $625 application fee, with an annual rental fee of CAD$3.50 per hectare.

Gemstone quality

The quality of gem ammolite is communicated via a letter grade system, from most desirable to least desirable: AA; A+; A; and A−. However, this system is not yet standardized and some vendors may use their own systems. The grade and therefore the value of an ammolite gemstone is determined by the following criteria:
; The number of primary colors : A large array of color is displayed in ammolite, including all the spectral colors found in nature. Red and green are far more common than blue or purple due to the latter's fragility. There are also certain hues, like crimson or violet or gold, which are derived from a combination of the primary colors, that are the rarest and in highest demand. The most valuable grades have three or more primary colors or 1–2 bright and even colors, with the lowest grades having one comparatively dull color predominant.
; The way the colors "play" : Chromatic shift is how the colors vary with the angle of viewing and the angle of light striking the gemstone. In higher grades this variation is almost prismatic in its scope, while lower grades show very little variation. Rotational range is how far the specimen can be turned while maintaining its play of color; the best rotate 360° uncompromised, while lesser stones may exhibit highly directional colors that are only visible within a narrow rotational range, down to 90° or less. Intermediate grades have ranges of 240°–180°.
; Brightness of colors and iridescence : The brightness of colors and their iridescence is essentially dependent on how well-preserved the nacreous shell is, and how fine and orderly the layers of aragonite are. The quality of the polish is also a factor. The "dragon skin" cracking usually hinders its value ; the most prized ammolite is the sheet type that has broad, uninterrupted swathes of color similar to the "broad flash" category of opal. The matrix is not visible in finer grades, and there should be no foreign minerals breaking up or diminishing the iridescence.
The thickness of the ammolite layer is also an important factor: after polishing, the ammolite is only 0.1–0.3 millimeters thick. The rarest and most valuable are thick enough to stand alone, with only a thin portion of its original matrix ; but the vast majority require some sort of supportive backing. Other treatments are also commonly undertaken; all other factors being equal, the less treatment an ammolite gem has received, the more valuable it is. Calibrated stones—that is, stones fashioned into standard dimensions that will fit most jewelry settings—may also command a higher price.