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Ammolite

2007 Schools Wikipedia Selection. Related subjects: Mineralogy


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   An unprocessed sample of ammolite on shale matrix, photographed wet and
   in natural light to simulate the effects of polishing. Typical of
   ammolite, reds and greens predominate, and a "dragon skin" pattern is
   apparent.
   Enlarge
   An unprocessed sample of ammolite on shale matrix, photographed wet and
   in natural light to simulate the effects of polishing. Typical of
   ammolite, reds and greens predominate, and a "dragon skin" pattern is
   apparent.

   Ammolite is a rare and valuable opal-like organic gemstone found
   primarily 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 mineral
   that makes up nacreous pearls. It is one of the three biogenic
   gemstones, the other two being amber and pearl.^1 In 1981, ammolite was
   given official gemstone status by the World Jewellery Confederation,
   the same year commercial mining of ammolite began. In 2004 it was
   designated the official gemstone of the Province of Alberta.

   Ammolite is also known as aapoak ( Kainah for "small, crawling stone"),
   gem ammonite, calcentine, and korite. The latter is a trade name given
   to the gemstone by the Alberta-based mining company Korite
   International, the first and largest commercial producer of ammolite.

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. Its crystallography is orthorhombic.
   Its hardness is 4.5–5.5, quite soft for a gemstone, and its specific
   gravity is 2.60–2.85. The refractive index of Canadian material (as
   measured via sodium light, 589.3 nm) is as follows: α 1.522; β
   1.672–1.673; γ 1.676–1.679; biaxial negative. Under ultraviolet light,
   ammolite may fluoresce a mustard yellow.

   An iridescent opal-like play of colour 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
   refraction, the iridescent colour 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, ca. 0.5–0.8
   millimeters (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 centimeters (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.

Formation, occurrence, and extraction

   Map of North America highlighting the shallow inland sea present during
   the mid-Cretaceous period.
   Enlarge
   Map of North America highlighting the shallow inland sea present during
   the mid-Cretaceous period.

   Ammolite comes from the fossil shells of the Upper Cretaceous
   disk-shaped ammonites Placenticeras meeki and Placenticeras
   intercalare, and (to a lesser degree) the cylindrical baculite
   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 Appalachain Mountains—this area is
   known today as the Cretaceous or Western Interior Seaway. As the seas
   receded, the ammonites were buried and crushed by layers of bentonite
   sediment. This sediment preserved the aragonite of their shelled
   remains, preventing it from converting to calcite. Through diagenesis,
   the shells were impregnated by trace elements present in these
   sediments, the most common of these being iron and magnesium, the
   former of which accounts for the predominance of the green colors.
   Korite International's mechanized mining operations are fairly basic,
   involving the excavation of shallow pits with backhoes.
   Enlarge
   Korite International's mechanized mining operations are fairly basic,
   involving the excavation of shallow pits with backhoes.

   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. 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 Lehigh River, in an area south of and
   between the cities of Summit Hill and Lethbridge. Roughly half of all
   ammolite deposits are contained within the Kainah (Kainaiwa) reserve,
   and its inhabitants play a major role in ammolite mining. Since its
   founding in 1979, Korite International has operated primarily within
   the reservation. The company maintains an agreement with the Kainah
   (Blood) tribe, with Korite International paying the tribe royalties
   based on how much land the company has mined.
   Another view of Korite International's open-pit mining operations in
   Alberta, Canada.
   Enlarge
   Another view of Korite International's open-pit mining operations in
   Alberta, Canada.

   Commercial extraction is mechanized but fairly basic: shallow open pits
   are dug with a backhoe and the excavated material is screened for its
   potential gem contents. The pits are further examined by hand, and
   commercial production is supplemented by independent persons who sell
   their surface-picked findings to Korite International. Approximately 5%
   of the ammolite mined is suitable for jewelry.

   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 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 the more
   valuable type of ammolite.

   As of 2003, Korite International had only mined 30 acres (120,000 m²)
   of the Kainah deposit. As part of their agreement with the tribe, the
   company must refill areas once exhausted, and ensure the environment is
   not adversely affected. Korite International's operations net the
   Kainah tribe approximately CAD $150,000–200,000 per year. The company
   employs over 60 people (many of them Kainah), and accounts for
   approximately 90% 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 best to least best: 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 colour 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 roughly equal portions of three or more
          primary colors or 1–2 bright and even colors, with the lowest
          grades having one comparatively dull colour 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 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 colour;
          the best rotate 360 degrees 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 (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
          (see formation) and has broad, uninterrupted swathes of colour
          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.

   Ammolite is considered the rarest organic gem material. It is soft and
   delicate, most often requiring special processing techniques known only
   to a few experts specializing in this commercial industry. In its rough
   state, ammolite is sold for USD $30 to 65 per carat (150 to 325 $/g).

Treatments

   Although fully mineralized and containing no water—and therefore not
   subject to dehydration and subsequent crazing as seen in opal—ammolite
   is often damaged due to environmental exposure. 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 International 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 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 onyx or glass
   could also be used as backing. In composites where the ammolite layer
   is exceptionally thin, a third component is used: this constitutes a
   triplet, with a durable and transparent convex topping piece. This cap
   may be either synthetic spinel, synthetic corundum, synthetic quartz,
   or in lower-end productions, glass. The convex cap acts as a lens and
   has the effect of enhancing the ammolite's iridescent display.

   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
   colour. 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

   The iridescent flashes (labradorescence) of labradorite may lead to its
   confusion with ammolite by the unfamiliar, but the overall appearance
   is unconvincing as an imitation.
   Enlarge
   The iridescent flashes (labradorescence) of labradorite may lead to its
   confusion with ammolite by the unfamiliar, but the overall appearance
   is unconvincing as an imitation.

   Ammolite is neither easily nor often imitated; however, a few materials
   have a passing resemblance that may deceive the unfamiliar. These
   include: labradorite (also known as spectrolite), an iridescent
   feldspar also of Canadian origin; and broad-flash black opal. Neither
   are convincing substitutes, and the latter is actually of greater value
   than ammolite. Indeed, ammolite is often used as an imitation of black
   opal. An even less convincing possibility is Slocum stone, a common
   glass-based imitation of opal. Blues and purples are much more
   pervasive in labradorite, and in both it and opal the play of color is
   seen to roll across the stone unlike the comparatively restricted play
   of color in ammolite. In Slocum stone, the play of colour takes the
   form of tinsel-like patches. The visible structure is also considerably
   different; in the imitations, the body of the stone is transparent to
   translucent from certain angles, whereas ammolite is entirely opaque.

   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 abalone (or paua;
   genus Haliotis) is one last possible imitation. Abalone shell is
   inexpensive and plentiful owing to the commercial mariculture of these
   gastropods for their meat. The shell's structure is distinctive:
   sinuous bands of blue, green, and rose iridescence are delineated by
   dark brown lines of conchiolin, a proteinaceous material that holds the
   shell together. The luster of abalone shell is silky rather than the
   near vitreous luster of polished ammolite, and the colors of the two
   materials do not closely approximate. However, some abalone shell has
   been dyed and given a transparent cap of synthetic quartz, forming a
   doublet in the same fashion as ammolite. These doublets are perhaps the
   most deceptive, and have also been used to imitate opal. Under
   magnification most abalone doublets will show dye concentrated along
   certain areas and air bubbles trapped at the shell-quartz interface.

Use in jewelry

   Fine ammolite jewelry by Korite International. The ammolite gems are
   triplets, as evidenced by their convex profiles, and are set in 14
   karat (58%) gold with diamond accents. Ammolite is best used in
   pendants, earrings, and brooches due to its fragility.
   Enlarge
   Fine ammolite jewelry by Korite International. The ammolite gems are
   triplets, as evidenced by their convex profiles, and are set in 14
   karat (58%) gold with diamond accents. Ammolite is best used in
   pendants, earrings, and brooches due to its fragility.

   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
   Blackfeet tribe know ammolite as iniskim, meaning " buffalo stone", and
   have long believed it to possess amuletic powers; specifically, the gem
   is believed to aid in the buffalo hunt, and to draw the buffalo within
   tracking distance. The Blackfeet also believe ammolite to possess
   healing powers and incoporate the gem into their medicine bundles for
   use in ceremonies.

   In the late 1990s, practitioners of Feng Shui began to promote ammolite
   as an "influential" stone with what they believe is the power to
   enhance well-being and detoxify the body by improving its flow of
   energy or " chi". Named the "Seven Color Prosperity Stone", each colour
   is believed by Feng Shui practitioners to influence the wearer in
   different and positive ways; a combination of ruby red, emerald green,
   and amber yellow is most sought after for this purpose, the colors
   being said to enhance growth, wisdom, and wealth, respectively.

   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.

   Japan is the largest market for ammolite; this may be due to its use as
   an imitation of increasingly scarce black opal, or its aforementioned
   use in Feng Shui. Secondary markets include Canada, where it is used
   both by artisans who sell their creations to tourists of Banff National
   Park and in fine jewelry production; and the Southwest United States,
   where it is used by Zuni and other Native American craftspeople.
   Retrieved from " http://en.wikipedia.org/wiki/Ammolite"
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