ALSO KNOWN AS: Korite or Calcentine
CHEMISTRY: CaCO3. Variable, may include aragonite, calcite, silica, pyrite, or other minerals
REFRACTIVE INDEX 1.52 – 1.68
HARDNESS 4.5 – 5.5
SPECIFIC GRAVITY 2.60 – 2.85
CLEAVAGE None, parting present
HEAT SENSITIVE Yes
WEARABILITY* Good to poor
SPECIAL CARE INSTRUCTIONS See article
ENHANCEMENTS Opticon and lacquer stabilization
*Wearability is graded as Excellent, Very Good, Good, Poor, and Forget It! For more details see the article on “Hardness and Wearability.”
In 1981, ammolite became recognized by CIBJO, (the Colored Stones Commission.) It is the latest of only three new gemstones introduced in the last 50 years. It is also only one of three organic gemstones, (including amber & pearl.) It has been compared to opal and has a superficial similarity to the Austrian mineral lumachelle whose iridescence is also provided by the fossil ammonite carnites floridus.
Ammonite shell is comprised primarily of aragonite, (the same mineral that makes up pearls,) with trace elements of aluminum, barium, chromium, copper, iron, magnesium, manganese, silicon, strontium, titanium and vanadium.
Unlike most other gems, whose colors 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 gems, less ordered thinner stacks for green and unordered, very thin stacks for blue ones.
The layers are .5 – .8 millimeters thick before polishing and .1 – .3 mm thick after polishing. This is sufficiently thick and durable to be manufactured into jewelry.
Ammolite is also known as aapoak, (Blackfoot for small, crawling stone,) calcentine, korite, ammonite shell or gem ammonite. Ammolite is the fossil shell of the upper cretaceous ammonites: Placenticeras meeki, Placenticeras intercalare & Baculites compressus.
Ammolite is 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 St. Mary river, south of Lethbridge.
75 to 70 million years ago, this was the northwestern 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 its inhabitants was the ammonite that thrived in every ecological niche, from deep-water bottom feeders to jet-propelled predators. It also had the biggest brain of its age. Cataclysmic volcanic activity from the growing rocky mountains periodically covered huge areas with meters thick ash that settled slowly to the shallow ocean floor. The ash killed all life below and sealed their remains in an heavy, semi-impermeable layer of mineral rich clay, (bentonite.).
Over time, the St. Mary river area was buried over 4 kilometers deep in ash. Through many glacial periods, it underwent geological uplift and eventually surface exposure. High concentrations of iron and magnesium created a regionally unique condition, which led to sedimentary digenesis. This impeded the oxidization process that converts the aragonite of the ammonite shell to stable calcium carbonate, (CaCO)3.)
Protected by either large ironstone concretions, (siderite or FeCO3,) or a thin layer of iron pyrite, (FeS2,) ammolite has formed in two distinct geological horizons known as the K Zone (crush) and the Blue Zone (sheet.).
The rate of infill of sea borne sediment into the ammonite chambers could explain the difference between crush and sheet material of iron pyrite.
K Zone – Always found in concretions, it has been compacted and fractured through deposition and naturally sealed with a carbonate or conchiolin. It is found 30 meters below the top of the Bearpaw formation.
Blue Zone – 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 collected in a horizon 120 meters below the bottom of the Bearpaw formation in river valleys.
The ammolite industry has yet to agree on any one grading system, however most systems refer to color, brightness, play of color, etc. We use the following chart:
|Colors||3 or more||1 or 2||1 or Pale||Dark / Pale|
|Iridescence||Brilliant||Bright||Included||Dull / Dark|
Gemstone ammonite can be found in any color known in nature or the imagination of man. It can also have an infinite array of color combinations in every square centimeter. The higher grades will either have a very strong, bright single color, or contain a range of bright colors drawn from a color spectrum vaster than a rainbow. 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, violet and gold, that are very rare and in high demand.
The property that allows us to see the incredible play of colors across its surface is iridescence. The shell’s surface is composed of closely packed, tabular, crystals of aragonite, with their c-axis oriented vertical to the shell surface. They are united into thin lamellae, (plates or scales.)
The thickness of these lamellae is of the same scale as the wavelengths of the spectral colors that 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.
The plates of aragonite crystals vary in thickness. In addition, the trace elements are randomly arranged and interspersed with inclusions of organic material, (conchiolin.) These three factors determine the colors and intensity of the diffraction. The best ammonite shell will have brilliant, vibrant iridescence, continually dancing with changing colors as the angle of the incident light changes.
The color of ammonite depends on the angle of light and the viewers perspective. Sometimes subtle and sometimes spectacularly dramatic, a chromatic shift occurs. Red will shift to green, 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, meaning the color will shift through the entire spectrum depending on light source and your angle of observation.
Some material will not show strong, vibrant colors through 360° degrees of rotation. As other gems are 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: naturals, doublets and triplets.
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 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.
Triplets are constructed of three layers. A dark gray 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 jewelry.
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 - Aerial map
Care & Handling
Naturals are best suited for brooches, pendants, or earrings. Triplets are best suited to rings. The care and cleaning of pearls also apply to ammolite. Ultrasonic and steam cleaners should not be used; rather, a commercial pearl cleaner or mild, warm soap is recommended, (to 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.
IGS gratefully acknowledges Tom Dryden, who has over 12 years experience in the Canadian ammolite industry, and is a recognized authority on the Ammonite Placenticeras Meeki and Intercalare. He founded the industry’s first contract cutting facility and currently works for Amgem International, Inc. Amgem is the world’s leading ammolite manufacturer and wholesaler. He contributed much of the text and many of the images for this work.
Also, our thanks to L. Allen Brown of “All That Glitters” in Methuen, MA for two of the Ammolite photos in this article.