Diamond Designs and More

Ammolites

 

Image of a ammolite Recognized by CIBJO, the Colored Stones Commission, in 1981, Ammolite is the latest of only three new gemstones introduced in the last 50 years and one of three organic gemstones (including amber & pearl). It has been compared to Opal and has a superficial similarity to the Austrian mineral Lumachelle whoís iridecsense is also provided by the fossil Ammonite Carnites floridus.

Also known as Calcentine, Korite, Aapoak (Blackfoot for small, crawling stone), Ammonite Shell or Gem ammonite, Ammolite is the fossil shell of the Upper Cretaceous Ammonites: Placenticeras meeki, Placenticeras intercalare & Baculites compressus.

Found only in the Bearpaw formation that extends from Alberta to Saskatchewan in Canada and 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 St. Maryís River south of Lethbridge.

75 to 70 million years ago this was the north-western edge of the Bearpaw Sea (also known as the Western Interior Seaway). It was a shallow warm water sea that teemed with life. One of the most prolific of itís inhabitants was the ammonite that thrived in every ecological niche from deep water bottom feeders to jet-propelled predators that had the biggest brain of itís age. Cataclysmic vocanic activity from the growing rocky mountains periodically covered huge ares with meters thick ash that settled slowly to the shallow ocean floor killing all life below and sealing their remains in an heavy semi-impermeable layer of mineral rich clay (Bentonite).

Over time the St. Mary River area was buried over 4 km deep and then underwent geological uplift and eventually surface exposure through many glacial periods. High concentrations of Iron and Magnesium in the volcanic ash created a regionally unique condition that caused sedimentary diagenesis that impeded the oxidization process that converts the aragonite of the ammonite shell to the stable calcium carbonate (CaCo2).

Protected either by large ironstone concretions (Siderite  or FeCO3) or a thin layer of iron pyrite (FeS2) Ammolite has formed in two distinct geological horizens known as the KZone (crush) and the BlueZone (sheet). Rate of infill of sea borne sediment into the ammonite chambers could explain the difference between crush and sheet material.of iron pyrite

Two zones

K zone - always found in concretions, it has been compacted and fractured through deposition and naturaly sealed with a carbonate or conchiolin. It is found 120 meters below the top of the Bearpaw formation.

Bluezone - sometimes in concretions, it is usually found compressed with a thin layer of iron pyrite. It is compacted with fewer or no fractures. Rarely mined it is hand collectedin a horizon 30 meters below the bottom of the Bearpaw formation in river valleys.
 

Bearpaw Formation Stratigraphic Section

Ammolite is made of Aragonite, the same mineral that makes up Pearls. Unlike most other gem, whoís color comes from light refraction, the iridescent color of Ammolite comes from interference with the light that rebounds from stacked layers of thin platelets in the aragonite. Ordered thick stacks for red gem, less ordered thinner stacks for green and unordred, very thin stacks for blue.

Sufficiently thick and durable and with the ability to take a polish to be manufactured into jewelry, the layers are .5 - .8 millimeters thick before polishing and .1 - .3 mm thick after polishing.


Gem characteristics

Mineral Name  Aragonite
Chemical Composition  Calcium Carbonate (CaCO3)
Specific Gravity  2.80 - 3.05
Refractive Index 1.52 - 1.67
Birefringence 0.135 - 0.145
Hardness 4.5 - 5.5
Morphology Crush or Sheet
Crystallography Orthorombic
Clarity Opaque or transparent or translucent in single layer
Luster Vitreous to resinous
UV fluorescence inert
Parting along flat layers in sheet material
Inclusions Pyrite between layers
Durability See below
Enhancements Opticon stabilization

Grading

The Ammolite Industry has yet to agree on any one grading system, however most systems make reference to color, brightness, play of color, etc. We use the following chart:
 
 
Grading

Gem Grade AA A+ A A-
Colors 3 or more 1 or 2 1 or Pale Dark / Pale
Iridescence Brilliant Bright Included Dull / Dark
Chromatic Shift Spectro Di Mono Little
Rotational Range 360° * 240° * 180° * 90°

Colors  Gemstone Ammonite can be found in any color known in nature and the imagination of man. Or it could have an infinite array of color combinations in every square centimeter. The higher grades will have either a very strong, bright single color or contain a range of bright colors drawn from a color spectrum more vast than a rainbow, while lower grade gemstones will show less vibrant colors in a more limited range. Generally red/green is more common than blue or purple, but there are certain hues, like crimson or violet or gold, derived from any of the primary colors that are very rare and in high demand.

Iridescence Ammonite shell is comprised primarily of aragonite with trace elements of aluminium, barium, chromium, copper, iron, magnesium, manganese, silicon, strontium, titanium and vanadium.     The spectral property of the mineral aragonite that allows us to see the incredible play of colors across itís surface is itís iridescence. The shell surface is composed of closely-packed, tabular, hexagonal crystals of aragonite oriented with their c-axis vertical to the shell surfaces and united into thin lamellae (plates or scales). The thickness of these lamellae is of the same magnitude as the wavelengths of the spectral colors which make up white light. Thus, when white light (sunlight) enters the regularly-spaced thin layers of aragonite, diffraction occurs, and flashes of spectral colors are seen. As the plates of aragonite crystals vary in thickness with the addition of the trace elements which are randomly arranged and is interspersed with inclusions of organic material (conchiolin) the intensity of the diffracted colors also varies. The best Ammonite shell will have brilliant, vibrant iridescence, continually dancing with changing colors as the angle of the incident light changes.

Chromatic Shift  The color of most Gemstone Ammonite changes dependent on the angle of light entering and the viewers perspective. Sometimes subtle, and sometimes spectacularly dramatic, a chromatic shift occurs. Most red will shift to green and most green will shift  blue, etc. This is called dichromatic. Some gem material will have the shift restricted to hues within the same primary color group. This is monochromatic. The best gem material has spectrochromatic shift. Color will shift through the entire spectrum depending on light source and your angle of observation.

Rotational Range With the iridescence and chromatic shift variations some material will not show strong, vibrant colors through 360° degrees of rotation. As the gemstone is rotated the brightness decreases and darkens to black. This is due to the light wave diffraction being blocked by the organic inclusions in the aragonite. For a gemstone to be Graded AA it must show a brilliant color through 360° of rotation.

Ammolite is sold in three forms: Triplets, Naturals and Doublets.

Triplets are constructed of three layers. A dark grey wafer of natural shale (1) sits below a layer of Ammolite (2). On top there is a calibrated cap of optical quartz or synthetic spinel (3). This assures that the Ammolite gemstone shows the most brilliant flash as well as being durable enough to be worn as every-day  jewellery.

Diagramm of ta triplet with its three layers

Naturals are free-form cabochons with a non-coated hand-finish. They are backed by the original shale of the fossil and can range in size from  10 - 15 carats to 6 inches across! Priced by carat weight the backing thickness should not exceed 1.5 mm.

Doublets are free-form cabochons that have been bonded to a backing.
 

Patterns

 Stained Glass - window panes
 Dragonskin  - scales
 Cobblestone  - regular, uneven rows
 Floral   - flower petals
 Ribbon  - long thin patterns
 Feather  - tendrils
 Tin Foil  - bright crinkle stack pattern
 Paintbrush  - broad strokes
 Moonglow  - inner glow, mono or di-chromatic
 Ripple  - regular striation lines
 Pinfire   - small plates of changing flash
 Sunset    - red tinged landscape scenes
 Lava River     - green with red rivers of lava
 Christmas tree    - green with red ornaments or freckles
 Suture Gem    - suture or leaf pattern
 Nipplites    - three dimensional rainbow eye or tubercle
 Ripple  - regular spaced rainbow striations or ribs
 Banding  - distinct color bands
 Stain  - spreading color changes
 Lava lamp  - color globules
 Terrain  - Airial map
 
 

Blue Zone - Ripple Blue Zone - Ripple Blue Zone - Ripple Blue Zone - Ripple
Blue Zone - Ripple Blue Zone - Ripple Blue Zone - Ripple Blue Zone - Ripple
Blue Zone - Ripple Blue Zone - Lava River Blue Zone - Lava River Blue Zone - Lava Lamp
Blue Zone - Ripple Blue Zone - Lava River Blue Zone - Lava River Blue Zone - Lava Lamp
Blue Zone - Terrain Blue Zone - Terrain Blue Zone - Nipplite Blue Zone - Nipplite
Blue Zone - Terrain Blue Zone - Terrain Blue Zone - Nipplite Blue Zone - Nipplite
Blue Zone - Nipplite Blue Zone Blue Zone Blue Zone
Blue Zone - Nipplite Blue Zone -  Blue Zone -  Blue Zone - 
Blue Zone Blue Zone Blue Zone Blue Zone
Blue Zone -  Blue Zone -  Blue Zone -  Blue Zone - 
Blue Zone Blue Zone Blue Zone Blue Zone
Blue Zone -  Blue Zone -  Blue Zone -  Blue Zone -
K Zone K Zone K Zone K Zone
K Zone -  K Zone -  K Zone -  K Zone - 
K Zone K Zone K Zone K Zone
K Zone -  K Zone -  K Zone -  K Zone - 
K Zone K Zone K Zone K Zone
K Zone -  K Zone -  K Zone -  K Zone - 

 

Care & Handling Naturals are best suited for brooches, pendants, or earrings. Rings should be made with triplets. Ammolite consists of aragonite. The care and cleaning for pearls also apply to Ammolite. Ultrasonic and steam cleaners should not be used; rather, a commercial pearl cleaner or mild, warm soap is recommended (a maximum of 20 minutes). Excessive heat, acids and some perfumes or hairsprays may cause damage or loss of iridescence.

Triplets may be cleaned, with caution, in ultrasonic cleaners. The cap will protect from scratches but care should be taken to avoid blows that could result in the separation of the bonded layers.

Sources:

Ammolite: Iridescent Fossilized Ammonite from Southern Alberta, Canada  - By Keith A. Mychaluk, Alfred A. Levinson, and Russell L. Hall - Gems & Gemology, Vol. 37, No. 1, pp. 4 - 25 - Spring, 2001 - Gemological Institute of America

Ammolite images and names supplied by Lorne Powell of Canada Ammonite Ltd.

 

 
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