About 75 million years ago in a shallow prehistoric sea, a ravenous reptile sank its peg-like teeth into a meter-wide shell of an ammonite. The ammonite fought for its life with its tentacles, but was no match the nine meter-long mosasaur. The crocodilian reptile extracted the mollusc's squid shaped body from its shell and devoured it. The punctured empty shell sank slowly to the sea floor, where it was eventually buried and crushed by tons of mud flowing in from a nearby river.
Today the lining of that shell has become one of the most unusual gemstones. Opalescent Aragonite. The word refers to ammonite, the name of the extinct animal that lived in the shell, and the gems brilliant display of reflected light
Ammonite is the smooth inner surface of the shell and as such, is really fossilized mother of pearl. But unlike modern mother of pearl, opalescent aragonite displays dazzling primary colors. Red, orange, yellow, green and blue flash across its flat surface. Each color is surrounded by thin black lines, creating a mosaic effect similar to a free formed stained glass window. Some stones are predominantly one color, others reflect the entire spectrum. Reds and oranges are the most common ammonite hues while blue is rare and more valuable. Although the ancient shells from which opalescent aragonite is derived are fairly common gem-quality pieces are difficult to find. Dr. Len Hills, professor of geology estimates that 10 billion fossilized shells in the shales of the Bearpaw Formation a vestige of the Bearspaw Sea which was a subtropical inland sea during the Late Cretaceous period about 75 million years ago.
Although 10 billion shells may seem like an ample supply, only portions of one in 100 shells are of of gem quality. Also the shells are buried at depths of up to 300 meters and their excavation is profitable only where the Bearpaw Formation lies close to the surface.
Rough pieces of the gem erode naturally from the river banks, creek beds and even wheat fields; exposed ammonite is easily bleached by the sun and damaged by the frost, and is usually of poor quality.
It took specific conditions to create the gem~, not all of them known. The ammonite would have had to die close to the shoreline of the Bearpaw Sea, near an inflow of fresh water. The water somehow changed the chemistry of the shell, possibly depositing iron on it, which accounts for the greens in the stone.
The shell had to have been crushed after burial. Although
whole shells are beautiful fossils in their own right, and may
even show broad planes of reds or browns where their
mother-of-pearl remains intact, they are never of gem quality. Opalescent aragonite's spectacular colors are the result
of the combination of elements that were added or taken away from
the original mother-of-pearl. Crushing allowed more reaction to occur, resulting
in more color.
Ammonite Renilda