Ammolite
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Ammolite | ||
---|---|---|
Specific gravity usually about 2.70 (varies with mineral content)[1] | | |
Polish luster | vitreous[1] | |
Optical properties | anomalous aggregate reaction[1] | |
Refractive index | usually 1.52 - 1.68 (varies with mineral content)[1] | |
Birefringence | 0.135 - 0.145[1] | |
Pleochroism | none[1] | |
Ultraviolet fluorescence | variable[1] |
Ammolite is an
Ammolite is also known as aapoak (
Properties
The
- Its orthorhombic.
- Its hardnessis 3.5–4.5 .
- Its specific gravityis 2.60–2.85 g/cm³.
- The refractive index of Canadian material (as measured via sodium light, 589.3 nm) is biaxial negative, with α = 1.522 ; β = 1.672–1.673 ; γ = 1.676–1.679 .
- Under ultraviolet light, ammolite may fluoresce a mustard yellow.
An
The ammolite itself is actually a very thin sheet, c. 0.5–0.8 mm (0.02–0.03 inches) 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. While these shells may be as large as 90 cm (35.5 inches) in diameter, the iridescent ammonites (as opposed to the pyritized variety) are typically much smaller. Most fossilized shells have had their aragonite pseudomorphously replaced by calcite or pyrite, making the presence of ammolite particularly uncommon.[4]
Origin
![](http://upload.wikimedia.org/wikipedia/commons/thumb/b/b3/IridescentAmmonite.jpg/250px-IridescentAmmonite.jpg)
![](http://upload.wikimedia.org/wikipedia/commons/thumb/2/25/Cretaceous_seaway.png/150px-Cretaceous_seaway.png)
Ammolite comes from the fossil shells of the Upper
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.[5]
Occurrence
![](http://upload.wikimedia.org/wikipedia/commons/thumb/d/d7/Ammolite_mining.jpg/250px-Ammolite_mining.jpg)
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.[4]
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
Since its founding in 1979, Korite has operated primarily within the reservation. The company had an agreement with the Kainah (Blood) 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
![](http://upload.wikimedia.org/wikipedia/commons/thumb/4/40/Ammolite_mining2.jpg/250px-Ammolite_mining2.jpg)
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 (generally speaking) 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.[5]
As of 2015[update],
Gemstone quality
![](http://upload.wikimedia.org/wikipedia/commons/thumb/6/6e/Ammolite_from_Placenticeras_fossil_ammonite%2C_Alberta.jpg/220px-Ammolite_from_Placenticeras_fossil_ammonite%2C_Alberta.jpg)
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:[5]
- 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 (see properties). 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 and rotational range)
- 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 prismaticin 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 [citation needed]; the most prized ammolite is the sheet type [citation needed] (see formation) 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 (not exceeding 1.5 mm); 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.
Treatments
Ammolite is often damaged due to environmental exposure, even though it is fully mineralized and contains no water — therefore it is not subject to dehydration and subsequent crazing seen in opals. The thin, delicate sheets in which ammolite occurs are also problematic; for these reasons, most material is impregnated with a clear epoxy or other synthetic resin to stabilize the flake-prone ammolite prior to cutting. Although the tessellated cracking cannot be repaired, the epoxy prevents further flaking and helps protect the relatively soft surface from scratching.
The impregnation process was developed over a number of years by Korite in partnership with the Alberta Research Council. Impregnated and epoxy-coated ammolite first entered the market in 1989 and the treatment significantly increased the availability and durability of the gem.
Because the ammolite layer is usually mere fractions of a millimeter in thickness, most ammolite gems are in fact composite stones: These usually take the form of two-part doublets, with the ammolite layer adhered to a dark backing material. This is usually the matrix or mother rock from which the ammolite was quarried; black
The detection of these treated and composite stones is relatively simple via inspection with a loupe; however, certain jewelry setting styles—such as those with closed backs—can complicate things. A triplet can be identified by inspecting the stone in profile; the top of the stone can then be seen to be domed and transparent, with no play of color. If the dome is made of glass, bubbles, swirl marks, and scratches may be present; the harder synthetic materials are optically flawless.
Although the vast majority of commercial-grade ammolite has been treated in some way, a small fraction of production requires no treatment other than cutting and polishing. Ideally, any treatments should be disclosed at the time of sale.
Imitations
![](http://upload.wikimedia.org/wikipedia/commons/thumb/5/5d/Labradorite_detail.jpg/220px-Labradorite_detail.jpg)
Ammolite is neither easily nor often imitated; however, a few materials have a passing resemblance that may deceive the unfamiliar. These include:
Gemologically speaking, ammolite can be grouped with the shell-based marbles. This group includes lumachella or "fire marble", a similarly iridescent marble composed of fossilized clam and snail shells. Found in Italy and Austria, lumachella is rarely if ever used in jewelry; rather, it is used as a decorative facing stone or in mosaics. The iridescence of lumachella is fragmentary and not nearly as brilliant as that of ammolite. Despite these differences, lumachella may be considered synonymous with ammolite in some circles.
The predominantly blue-green iridescent shell of
Use in jewelry
![](http://upload.wikimedia.org/wikipedia/commons/thumb/7/78/Ammolite_jewellery.jpg/250px-Ammolite_jewellery.jpg)
Compared to most other gems, ammolite has a rather scant history of use; it did not begin to garner interest in Western society until the 1970s after entering the market (to a limited degree) in 1969. The
In the late 1990s, practitioners of
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; if used as a ring stone, ammolite should be given a hard protective cap, namely one of synthetic spinel as used in triplets. Whole polished ammonites of appropriately small size may also be mounted in jewelry. Nothing harsher than mild soap and warm water should be used to clean ammolite jewelry; ultrasonic cleaning should be avoided.
Footnotes
![](http://upload.wikimedia.org/wikipedia/en/thumb/4/4a/Commons-logo.svg/30px-Commons-logo.svg.png)
- ^
Ammolite gemstones are distinguished from other organically produced, precious ornamental materials, such as tortoiseshell, etc., primarily by the ways in which they are used. Pearls, amber, and ammolite are considered to be jewels,[citation needed] while the others are treated more as textural objects.
References
- ^ ISBN 978-0-87311-019-8.
- ^ "Ammolite". Jewellery Business Magazine. June 2007. Archived from the original on July 3, 2007. Retrieved July 20, 2007.
- ^ "Minutes of City Council Meeting" (PDF). City of Lethbridge, Alberta. April 30, 2007. Retrieved December 5, 2007.[dead link]
- ^ ISSN 0048-8453.
- ^ doi:10.5741/GEMS.37.1.4. Archived from the original(PDF) on October 5, 2016. Retrieved January 11, 2015.
- ^ "Alberta Energy: Error - Page can not be found". www.energy.gov.ab.ca. Archived from the original on December 27, 2013.
External links
- Research and development at the Alberta Research Council (Report). Edmonton, AB: Alberta Research Council. Summer 2004. p. 7.
- Pedersen, Maggie Campbell (2004). Gem and Ornamental Materials of Organic Origin. Elsevier. ISBN 978-0-7506-5852-2.
- Hoggan, Kyra (December 23, 2003). "Blood Tribe building economic growth for future generations". Aboriginal Times (aboriginaltimes.com). Archived from the original on March 24, 2005. Retrieved February 18, 2005.
- "Ammolite jewelry and gemstone information". gemsociety.org. International Gem Society. Archived from the original on December 24, 2013. Retrieved December 23, 2013.
- Li, Edward Kui Ming. "The Most Influential Stone of the Millenium". Korite. Archived from the original on November 14, 2012. Retrieved December 23, 2013.
- O'Donoghue, Michael (1997). Synthetic, Imitation, and Treated Gemstones. Gulf Professional Publishing. ISBN 978-0-7506-3173-0.
- Gemstones of the World. Sterling Publishing Company Incorporated. 1997. ISBN 978-0-8069-9461-1.
- Susan Ward Aber. "Gems in Brief (A-C)". Emporia State University. Archived from the original on December 24, 2013. Retrieved December 23, 2013.
- Webster, Robert (1994). Gems Their Sources, Descriptions, and Identification. Digital Press. ISBN 978-0-7506-1674-4.