Ammnoidea and Ammolite: A Journey
of One of the Rarest Organic Gems
in the World

by Bud Williams

Iridescent ammolite shell. Image taken from Photo by
Gregory Phillips, 2/2005

An early naturalist, Pliny the Elder, was the first to name the newly discovered fossilized shells Ammonis cornua or the horns of Ammon (Amun) the Egyptian god, who is often depicted wearing ram’s horns.(1) This was in 79 B.C., half a world away from my home in Canada. An interesting fact is that today, the ammonoids of Alberta, Canada had their shells preserved as I shall describe later, and these have become valued as an extraordinary gemstone and remain one of the rarest organic gems in the world.(8)

This webpage report was done for partial completion of a gemstone course, taught at Emporia State University, in Kansas, by Dr. S. W. Aber. The assignment of my choosing was to learn and report on ammolite, its geologic place in history and use today as a gemstone.

Taken from
Photo by DanielCD, 4/19/05.

Table of Contents


Ammonoidea are found within the class cephalopodia, whose last surviving member is the Nautilus. Nautilus live in the tropics; specifically, the ocean waters off the warm coastal waters of the Indio-Pacific islands.(2) Fortunately, the Nautilus is very closely related to the ammonoidea in which I am interested. The purpose of this report is to follow the journey of an ancient organism to a valued gemstone that serves as the province gem of Alberta, Canada. We can better understand the fossilized organic gem, if we are able to see how modern Nautilus was constructed and lived because the differences are not very marked from their ancestors who lived during the geologic time period, the Cretaceous. We can use what we see in the present to interpret the past life of the Cretaceous-age ammonoid who lived and died in a shallow sea that stretched from Kansas to Calgary. The organism was buried and fossilized, and now it is mined in the southern part of Alberta.

Click on image for larger view.
Image scanned from R. Black.

Ammonoidea and Nautilus

To the right is a drawing of the Nautilus from Rhona Black’s book, The Elements of Paleontology. This scanned image, shows clearly how these animals were made. The smallest chamber is at the center, and called the protoconch; many conches formed beyond this center chamber. The septa are convex forward, and each is perforated centrally by a tube, the siphuncle with a septal neck (Black, p.84).

The animal itself lived in the largest conch. It had two well developed eyes, like the modern-day octopus. Interestingly, the eyes of an octopus are very similar to the human eye. The speculation is that ammonoidea had a soft body, and around the mouth are retractable tentacles. Although there were similarities, differences exist as well. Nautilus has jaws, which were different in the ammonoid. Instead of jaws, the ammonoid had for a lower plate called an aptychi, which were calcite plates presumed to serve either as the jaw or opercula.

Click on image for larger view.
Image scanned from R. Black.
In life, the aptychi, the lower jaw and small upper horny jaw, had occasionally a radula lying between them. This latter is a toothed tongue. In the drawing, you can see the funnel. This can squirt out water, as a defensive mechanism, or just to change position, and direction. The conches in life are filled with gas, mostly nitrogen.(2) The animal could change it level in the water, by varying the amount of gas. They lived at depths of 5 to 500 m. Over 800 m the shell collapses from water pressure.

Since it is known that Nautilus had sexual dimorphism, it was suggested that the ammonoids also had this. The female has a larger conch and the macroconch, to accommodate the young. The male has a smaller one, the microconch.(1)

In the book, Oceans of Kansas by Michael Everhart, he mentions the ammonites of the Smoky Hill Chalk. The aptychi of the ammonoids are photographed – see my print of them to the right. There seems to be no real ammonoid shell there, just fragments. It is presumed that the aragonite dissolved.(3) How fortunate it was that in Alberta, the aragonite was preserved and formed our beautiful gem.

Image taken from

Ammonoidea and Ammonite

Ammolite, the gemstone, is the remains of the shells of the ammonoids, Placenteris meeki and Placenteris intercalare. The identifying feature for the Placenteris intercalare is the raised tubercles, that are evenly spaced in two concentric rings along from the protoconch to the edge of the living chamber. Some feel that the Placenteris intercalare are males, and some that the Placenteris meeki were females. These latter are hard to find, because many think that they died shortly after spawning.

Image taken from

They lived in shallow waters in the Great Inland Sea. See the image to the right from Wikepdia, where it is referred to as the Western Interior Sea. (9.) This shallow sea divided North America, from the Gulf of Mexico to what is now the Arctic Ocean. This was a relatively warm shallow sea and coastlines because it was close to the Equator of its day during the Cretaceous geologic time period.

In the various mass exterminations previously, the cephalopods were often eliminated, except for a few that survived each of the episodes.(7.) Like the dinosaurs, they all died out except for the last of the cephalopods, the Nautilus. The ammonoids were about two feet in diameter, but I recall, on going through the back rooms of the Royal Tyrrell Museum (, there was a cephalopod about six feet across. Its shell was covered here and there with rock, so we could not tell what kind it was. These particular ammonites varied from several inches in size, to two or more feet in diameter. The sea that they lived in was called the Bearpaw Sea. Here the ammonites had many rivals, and there are tooth marks in some of the shells suggesting mososaurs, a sea-living relative of the dinosaurs.

Click on for larger view.

Click on for larger view. Scanned from R. Black.
There were many volcanic eruptions during this time – coming from the Cascade mountains, from Mt Shasta to Mt. Garibaldi. These inundations of ash, solidified as bentonite, and periodically trapped the animals, sealing their remains. These ashes became meters thick, even up to 4 km thick during one episode. (8.) The combinations of iron, magnesium and other minerals protected the delicate aragonite from being destroyed, as in Kansas.
As erosion progressed, there were some of these would become freed, some of these shells became visible, and the local Indians collected them. They were named by a local rock-shop owner in Calgary, Marcel Charbonneau (alias Mugs), and in 1981 became an officially recognized gemstone. (8) The stones are made of aragonite, an unstable form of limestone. These were protected by the deep burial. Mining, on commercial basis, has been going on between the town of Magrath and Lethbridge. Most of the gems come from the Indian reservation, by an agreement with the tribe and Korite International, paying royalties to the tribe. (8)


Some of the preserved ammonites are gemstone quality and now referred to as ammolites. The number of primary colors displayed in ammolite includes all spectral colors found in nature. Red and green are much more common than the blue or purple, because of the fragility of the aragonite scales. Crimson and gold are the rarest, bring formed by combinations of primaries. All of these colors are like those of my diffraction spectroscope.

The colors show chromatic shift, depending upon the angle of light striking the gemstone, almost prismatic in scope. Rotational range is how far the stone can be turned while maintaining its play of color – the best is 360 degrees. Some are down to 90 degrees, the intermediate ones have 240 to 180 degrees. The brightness of colors and their iridescence depends upon how well preserved the nacreous shell is, and how fine the orderly layers of aragonite are. The damaged ones, dragon skin, as seen in the title image, hinders the value – the best has broad, uninterrupted swathes of color, similar to the broad flash category of opal. (8)

In 1981, ammolite became recognized by Confédération International de la Bijouterie, Joaillerie, Orfèvrerie des Diamantes, Perles et Pierres, which translates to International Confederation of Jewellery, Silverware, Diamonds and Stones (CIBJO). It is the latest of only three new gem stones introduced in the last 50 years. (8) Unlike gems whose color comes from light refraction, the iridescent color comes from interference with light that rebounds from stacked layers of thin platelets of aragonite.

Image taken from Korite International
More information is available at
The thickness of the ammolite layer is an important factor, usually less than 0.1 to 0.3 mm thick. The rarest has its own backing of the original matrix (not exceeding 1.5 mm), but the vast majority require some backing. Ammolite is the rarest organic gem material. (8) It is soft and delicate. The gems are quite thin, formed of aragonite in very thin layers which refract and diffract the light. By this means, the light becomes separated into all the colors of the rainbow. Since they are very thin, most are sold as duplexes or triplets. The unprotected ones are not commonly sold.
Ammolite is variable in its chemistry and includes aragonite, calcite, silica, and pyrite within a shale matrix. The specific gravity is 2.6-2.85. It is often placed in jewelry with a doublet or triplet assembled stone as is opal,, to protect the delicate stone.

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  1. Ammonite (n.d.). Wikipedia. WWW URL:
  2. Black, Rhona H. (1988). The Elements of Paleontology, 2nd edition. Cambridge University Press. p.69, 79, 81, 84.
  3. Everhart, Michael J. (2005). The Oceans of Kansas. Indiana University Press, p.36.
  4. Espy, S. (2003-2009). Espy Jewelry WWW URL: , p.4.
  5. Aurora Ammolite Mine (2003). WWW URL:
  6. Ammolite (n.d.). American Gem Society. WWW URL: www.gemsociety/org/info/gems/ammonite.htm
  7. Prothero, D.R., and Dott, R.H. (2003). History of the Earth. McGraw Books.
  8. Ammolite (n.d.). American Gem Society. WWW URL: www.gemsociety/org/info/gems/ammonite.htm
  9. Ammonites (n.d.). Cretaceous Seaway.png. Wikipedia. WWW URL:

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