WHAT IS AN AMMONITE? WHAT IS AMMOLITE?

The term Ammonite refers to an extinct group of mollusks. These creatures, along with the dinosaurs, died out at the end of the Cretaceous Period about 65 million years ago. Their existence on earth lasted for 330 million years. 

When Ammonites died, they were deposited at the bottom of the ocean and covered with sediment. After being buried for million of years, the remains of these marine organisms fossilized and left behind shells which are found throughout the world. Some of the fossils served as a nucleus for the formation of concretions. In Southern Alberta, the shells have an outer layer of beautiful colours since local geological and mineralogical processes transformed the mother of pearl into a gem called Ammolite. The mineral that forms the color is known by aragonite. Ammolite is mined in Lethbridge, Alberta, the only area known to yield gem-quality Ammolite in commercial amounts. 

Ammonites with iridescent material (Ammolite), have also been found outside of Alberta. Areas in Montana, Saskatchewan, Utah, England, Morocco and Madagascar do contain colored fossils. However, when compared to Ammolite from Alberta, their color is weak. Most fossils outside the formation in Alberta are usually too thin to produce gems, too patchy in color and again not bright and vibrant enough to be considered gem-quality Ammolite.

 

Stratigraphy And Fossil Locations

Around 70 to 75 million years ago (during the Campanian Age or Late Cretaceous period), rain falling on the young Rocky Mountains washed sediment into the Western Interior Seaway (a sea way that ran across North America, from the Arctic Ocean down to the Gulf of Mexico). The sediments accumulated and formed a rock unit. This unit is called the Bearpaw Formation which is composed of marine shale, sandstone and some volcanic ash. Ammonites are also found within this formation in dark grey layers of pyritic shale.

Some of the most rare and beautiful colored fossils are found in the Blue Zone, which is situated at the bottom 150ft of the Artica Ovata Zone. Fossils from the Blue Zone are about 83 million years old. There is a scarcity of fossils in this layer because there is no protection from iron stone concretion as is the case in other zones. This means that many fossils from the Blue Zone are unsuitable for restoration and are often incomplete. They usually require reassembly because the tectonic pressure from the layers of marine sediments broke the shell before mineralization replaced and strengthen the original organic material. This phenomenon explains why there are less Blue Zone Ammonites on the market and why their value can be quite high. 

 

Iridescence

Ammonite shells are mostly made up of aragonite with some traces of aluminium, barium, chronium, copper, iron, magnesium, manganese, silicon, strontium, titanium and vanadium. What allows the human eye to see a play of colors on Ammolite is its spectral properties of iridescence. The shell contains closely packed. tabular, hexagonal crystals of aragonite oriented with their c-axis vertical to the shell surfaces and united into thin scales. The thickness of these lamellae (plates or scales) is of the same magnitude as the wavelengths of spectral colors that make up light. Therefore, when light enter the layers of aragonite, diffraction occurs and flashes of spectral colors are seen. 

Variation in thickness of the aragonite plates, different trace elements and the addition of organic material (conchiolin), are factors that will affect the intensity of the colors seen. High quality stones will be very bright and show iridescence with changing colors as the angle of the light changes. 

Chromatic Shift

 Lower quality stones will show a subtle change in colors as the angle of light entering the layers of aragonite changes while higher quality Ammolite will show a dramatic chromatic shift. Chromatic shift is when a color, such as red, shifts to green. Green then usually moves to blue. Monochromatic shifts are when a gem's chromatic shifts is restricted within one primary group color. Dichromatic shift is when a stone's color, like red, shifts to green. The best Ammolite will show spectrochromatic shift; where the stone can shift throught the entire spectrum, depending on the angle of observation.

One way to measure chromatic shift, is by using a rotational range. High quality stones will be able to complete a 360 degree rotation and have the colors appear bright the whole way. Lower grades of Ammolite, will darken as they are rotated. This phenomenon is caused by the blocking of light diffraction from organic inclusions in the the aragonite. 

(some information used has been gathered from www.ammonite.com)