Ammolite
Ammolite is a rare opalescent gem stone . It is found mainly on the eastern slopes of the Rocky Mountains and consists of the fossil remains of ammonites . Ammolite is also offered under the trade names Calcentin or Korit . In the languages of the indigenous there Blackfoot - Indian tribes , the stone is Aapoak ( small, crawling stone in the language of Kainai due to the play of color) or Iniskim called ( "buffalo stone").
Emergence
During the Cretaceous North America was a large subtropical inland sea, the Western Interior Seaway . Continental drift caused the younger part of the Rocky Mountains to unfold as the sea gradually disappeared. The ammonites Placenticeras meeki and Placenticeras intercalare and, more rarely, Baculites compressus lived in this warm sea .
Their shells sank to the sea floor after death and were covered by clay ( bentonite ). In these bentonite sediment layers, the ammonites were largely crushed, but the shells were preserved. These bowls were partly made of mother-of-pearl , i.e. fine, platelet-like aragonite crystals that are embedded in a protein matrix.
In most of the petrified shells, the aragonite was leached out because it is more easily soluble than, for example, calcite in the surrounding rock. The resulting cavities were later often filled with other materials such as calcite or, more rarely, pyrite , or remained hollow. At temperatures above 400 ° C, the more unstable aragonite turned into the more stable calcite. Therefore, fossils with preserved aragonite are particularly rare.
In the case of the ammonite casings from which ammolite was made, the process was somewhat different. The aragonite remained, this was mainly due to the cover by impermeable volcanic ash , which came from eruptions of the volcanoes of the forming Rocky Mountains. At the same time, the ammonite-containing layers did not get too deep, so that they did not heat up to over 400 ° C. During diagenesis , trace elements such as iron and magnesium migrated into the shells.
Only rarely are completely well-preserved ammonites found in which the praised lines can still be seen. Ammonites up to 90 cm in size have been found, but typically the opalescent ammonites are much smaller.
properties
Ammolite consists mainly of aragonite, which comes directly from the original mother-of-pearl in the shells of ammonites. In addition to aragonite, calcite, quartzite , pyrite and other minerals occur in variable proportions . A number of trace elements occur in the shell itself (aluminum, barium, chromium, copper, iron, magnesium, manganese, strontium, titanium and vanadium).
Ammolite is (like aragonite) in the rhombic crystal system , but shows a somewhat greater hardness 4.5 to 5.5 and density 2.60 to 2.85 g / cm³ than this. Ammolite is optically biaxial , the refractive indices of the Canadian material (at 589.3 nm, yellow sodium light) are: α 1.522; β 1.672 to 1.673; γ 1.676 to 1.679, and optically negative . Some ammolites show a mustard-yellow fluorescence under UV light .
Due to the layer structure of the aragonite obtained from the original mother-of-pearl, there is an opalescence. This is caused by the interference in thin layers, such as in soap bubbles or oil stains, not by a natural color or refraction. The color depends on the layer thickness: thick layers produce red and green interference colors, thin layers also produce blue and yellow tones.
Like the original shell, the ammolite is only present in a very thin (0.5–0.8 mm) layer, which is mostly on a gray-brown matrix of slate , marl limestone , or limestone . The rock was compressed by overlying layers of sediment, which is why the ammonites are mostly crushed, so that countless cracks form in the thin layer. This crack texture is sometimes described as dragon skin or church glass window-like . Ammolite from deeper layers can also be completely smooth or have a ripple-like surface.
Locations
Ammolite is found primarily in the Bearpaw Formation , which stretches from the Canadian provinces of Alberta and Saskatchewan to Montana in the United States. The best quality gems can be found here on the eastern slopes of the Rocky Mountains in southern Alberta.
A similar material is Lumachella , a marble made from petrified shells and snails found in Italy and Austria. Lumachella only partially opalesces and is not as brilliant as ammolite due to the light-colored substrate. The marble is therefore not or only rarely used for decorative purposes. Like other marble, it is used in mosaics or as cladding panels.
Most of the commercial ammolite mines are located in the valley of the Saint Mary River , a tributary of the Oldman River , in the area of the towns of Cardston and Lethbridge . The reserve of the Kainai Indians is of great importance , where about half of all ammolite deposits are found.
Ammolite can also be found in Madagascar , Russia and Somerset / England .
Extraction in the Kainah reservation
The most important company for the commercial extraction of the ammolite is Korite International , which has worked mainly in the Kainah reservation since its foundation in 1979. The society pays the tribe a usage fee for the areas it has mined.
The extraction takes place via shallow opencast mines that are excavated with excavators. The extracted material is searched for its possible ammolite content. The pits are also searched directly by non-employees for surface ammolites which are subsequently sold to Korite International . Almost five percent of the mined ammolite is suitable for processing into jewelry.
The ammolite deposits are stratified: the uppermost of these strata, called the K-zone , lies approx. 15 m below the surface and extends 30 m into the depth. The Ammolite within this layer is by siderite - concretions surrounded and is usually broken with an amount Committee. It is the most common and generally the least valuable ammolite.
The Blue Zone is then twenty meters below . In this 65 m thick layer, the ammolite is usually surrounded by a thin layer of pyrite instead of siderite. This ammolite forms rather flat layers here that are less broken. This ammolite is the most valuable due to its better quality and the rarer extraction due to its great depth.
By 2003, Korite International had only mined 30 acres of the Kainah deposit. As part of their agreement with the tribe, the company must replenish the exploited areas and ensure that the environment is not permanently adversely affected. The company employs about 60 people, most of them are Kainah the tribe falls per year, about a net profit 150000-200000 CAD $ off. Around 90 percent of the world's annual production of ammolite gemstones is obtained here.
Use as a gem stone
With amber and pearls, ammolite is one of the biogenic gemstones. In 1981, together with a market launch of the previously unknown stone, mining began in a larger open pit mine by the mining company Korite International . In 2004, ammolite was recognized as an official gem of the Canadian province of Alberta .
Ammolite is considered to be the rarest organic gemstone. In its raw state, it is priced from $ 30 to $ 65 per carat ($ 150-325 / g). As it is soft and sensitive, it requires special processing that only a few experts are familiar with. This processing makes the stone more resistant and more colorful. In addition to cutting into the desired shapes, the stone must be polished as a rough surface scatters the light too much and therefore only shows dull colors. A coating with a resin stabilizes the thin flakes in which ammolite is present.
history
Compared to most other gemstones, ammolite has a short history: The stones did not attract greater interest until the 1970s, after they were launched in small quantities in 1969. The Kainah tribe knew the stone as Iniskim ( buffalo stone ) and for a long time believed that the stone had magical powers that aid in hunting buffalo and attract buffalo. In addition, the stone should have healing powers, so the stones were used in medicine man ceremonies .
In the late 1990s, Feng Shui practitioners began promoting ammolites as influential . The stone is said to have the power to increase well-being and detoxify the body by improving the flow of energy chi . The so-called dragon scale (English Seven Color Prosperity Stone ) is said to influence the wearer in various positive ways with each color: a combination of ruby red, emerald green and amber yellow is therefore in great demand, the colors are supposed to promote growth, wisdom and health.
Japan is the largest market for ammolite. One reason for this is the replacement function for the too rare black opal, the other is its use in Feng Shui . The second most important market is Canada: Ammolite is used both by artists who sell their creations to tourists in Banff National Park and by jewelers. It is used by the Zuñi and other Native American artisans in the American Southwest .
processing
In contrast to opals , ammolite is completely mineralized and contains no water. Therefore, like the opal, it cannot dry out and form cracks. Nevertheless, ammolite is also quickly damaged by the environment. This is mainly due to its low hardness. Even normal dust with tiny grains of sand (= quartz with a Mohs hardness of 7) can cause scratches. The fact that the ammolite is only present in thin, sensitive leaflets increases the risk of damage from flaking off the leaflets.
Only a small part of the Ammolite is of such a high quality that it does not need any treatment other than cutting and polishing. Most stones are impregnated with epoxy or other synthetic resins to stabilize them before cutting, so that the sensitive ammolite layer does not flake off. This can prevent new cracks from forming, but existing cracks can no longer be healed. In addition, the coating is a protection against scratching. This impregnation technique was developed over several years by Korite International in collaboration with the Alberta Research Council and was first introduced to the market in 1989, which means that significantly more ammolites are available as gemstones.
After polishing, the ammolite layer is only 0.1 to 0.3 mm thick. Only particularly rare and valuable ammolites are still thick enough to be sufficiently stable together with a thin layer (less than 1.5 mm) of the original matrix and thus to be used as a gemstone. Most stones, on the other hand, still need to be stabilized by combining them into doublets . The thin ammolite layer is placed on a dark, stable carrier. The carrier can either be the dark matrix in which the ammolite is usually embedded, but also black onyx or glass. In the case of particularly thin ammolite layers, the duplicates are expanded: a hard, transparent cover is glued onto the ammolite. This usually convex cap can consist of synthetic spinel , corundum or quartz or just glass. A convex cap acts as a lens and enhances the angular range in which the play of colors can be observed.
Ammolite is usually designed in free-form cabochons and set in gold, with diamonds creating accents. Because of its sensitivity, ammolite is ideally only used in pendants, earrings and brooches. As a ring stone, ammolite should only be used in triplet form with a solid cap (e.g. spinel). But enough small, completely polished ammolites are also used as jewelry. Only warm water with a mild soap should be used for care, ultrasonic baths can destroy the stone.
Quality designations
The quality of ammolites that are used as gemstones is rated using a letter system from good AA to A + , A to bad A- . However, this system is not yet standardized and some vendors use their own systems. This classification, and consequently the value of an ammolite gemstone, is determined by the following criteria:
- Number of colors
- Ammolites show a variety of colors, but most only red and green. Blue, purple or yellow are much rarer, which is why they are more valuable. The most valuable have roughly equal proportions of three colors or one or two bright, even colors. Stones that are predominantly matt in color are less valuable.
- Play of colors
- The color of the ammolite changes when viewed from different angles. With high-quality stones, the colors change in a wider spectrum. In addition, the angular range in which the opalescence can be observed is different: very good stones show their play of colors in a range of 360 °, poorer qualities only opalesce in a limited angular range of 90 ° or less.
- brightness
- The brightness essentially depends on how well the former mother-of-pearl layer is preserved and how well the aragonite platelets are arranged. The polish also determines the brightness.
- surface
- A smooth, not torn ammolite layer is most valuable, cracks reduce the value. The matrix or other foreign minerals should not be visible so that wide, uninterrupted bands of color can run over the ammolites.
- Build quality
- The value of the ammolites also depends on the quality of the processing. The less additional treatment is required, the more valuable the gemstones are. Proof that the ammolites have been doubled or tripleted is usually easy to do with a magnifying glass, but it can be more complicated with set stones.
Imitations
Ammolite cannot simply be imitated, nor does it happen frequently. In contrast, it is offered as an imitation for the even more valuable black opal . However, some other substances show similarities. The labradorite shows a bluish opalescence. The blue and purple, however, are more intense than that in ammolites. Opal imitations like the Slocum Stone show a play of colors in smaller tinsel. Another distinction is that ammolite itself is opaque, in the possible imitations the material is more translucent.
One possible imitation is the mother-of-pearl of the abalone . This snail, which is preferably green-blue opalescent, is cheap because it is collected in large quantities for consumption. The shell structure is striking, however: curved, blue, green and pink opalescent bands are delimited by dark brown lines made of conchiolin . The sheen of this mother-of-pearl is rather silky compared to the more glass-like sheen of polished ammolite. The colors do not correspond either. Nevertheless, there are ammolite imitations made of colored abalone mother-of-pearl doubled with a cap made of synthetic quartz. These imitations are the most deceptive imitations and are also made as opal imitations. Under the microscope, however, accumulations of the dye and air bubbles between the quartz and mother-of-pearl can be seen.
literature
- Research and development at the Alberta Research Council. Alberta Research Council, Alberta 2004, ISSN 0830-1093 , p. 7.
- M. Campbell Pederson: Gem and ornamental materials of organic origin. Elsevier Butterworth-Heinemann, London 2004, ISBN 0-7506-5852-5 , pp. 224-225.
- M. O'Donoghue: Synthetic, imitation & treated gemstones. Elsevier Butterworth-Heinemann, London 2002, ISBN 0-7506-3173-2 , p. 149.
- W. Schumann: Gemstones of the world. Sterling Publishing, London 2000, ISBN 0-8069-9461-4 , pp. 48, 240.
- R. Webster, PG Read: Gems. Their sources, descriptions and identification. 5th edition Butterworth-Heinemann, London 2000, ISBN 0-7506-1674-1 , pp. 299, 317, 569.
Web links
- Korite International (English)
- Many pictures of crack textures (English)
- Importer for Europe
