Ammolite: The Fossil with the World's Most Spectacular Iridescent Colors

Write By: tmmadmin Published In: Mineral Species Created Date: 2016-06-17 Hits: 5377 Comment: 0

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WHAT IS AMMOLITE?

Ammolite is an organic gemstone made from the nacre-like shells of fossilized ammonites which are found in Alberta, Canada. Its primary feature is its brilliant, iridescent colors, which are result of the material's opal-like qualities. Ammolite is mainly composed of aragonite, CaCO3. It is also known as aapoak, gem ammonite, and korite (a trade name). Ammolite is found on found on the shells of fossil ammonites from the Upper Cretaceous period (135 million years old). Ammolite is rare, occurring in only one small area near St. Mary River in southwestern Alberta, Canada.  

 

WHAT CAUSES THE COLORS IN AMMOLITE?

Ammolite consists of a very thin sheet, only about 0.5 to 0.8 millimeters thick. The iridescence is due to the organic microstructure of the aragonite, which causes light to rebound from stacked layers of thin platelets. This is different from any gemstones with iridescence. Iridescence is typically a result of light absorption, but ammolite color is a result of light interference. Thicker layers produce reds and greens, while thinner layers result in blues and violets. Because thin layers are fragile, reds and greens predominate. The colors are not very strong when the material is mined – polishing and other treatments are required to bring out the vivid coloring.

Color play (iridescence) is determined by chromatic shift and rotational range. Chromatic shift refers to the angle of view and the angle of the light. High grade ammolite has excellent chromatic shift, so the colors appear brighter and more iridescent. Rotational range means how far a specimen can be turned without lessening the color intensity. The best pieces have 360-degree rotational shift.

Color brightness and iridescence are also affected by factors including how well-preserved the nacreous shell is, how orderly the layers of aragonite are, and the quality of the polish. Crackling in the shell reduces the size of the areas that flash, and visible matrix (shale, clay or limestone) reduces the visual appeal as well.

WHERE IS AMMOLITE FOUND?

Gem-quality ammolite is only found in the Bearpaw Formation on the eastern slopes of the Rocky Mountains in southern Alberta, Saskatchewan and northern Montana. Commercial mining takes place in an area between the Magrath and Lethbridge in Alberta, on the banks for the St. Mary River. Half of the known reserves are on 30 acres of the Kainah tribal reservation, which receives royalties for the material mined. Mining is done using backhoes, with hand picking. The best material is found between 20 and 65 meters deep, in a thin layer, usually only a few feet thick. Ammolite from shallow pits is usually frost-shattered, which results in a cracked, "dragon skin" appearance. Ammolite from deeper excavations is often smooth, or has a rippled surface.

 

WHERE DID AMMOLITE COME FROM?

Ammolite comes mainly from the fossil shells of two extinct species of disk-shaped nautilus-like fossil ammonites: Placenticeras meeki and Placenticeras intercalare. Ammonites were cephalopods that lived in an inland subtropical sea on the eastern side the Rocky Mountains. They became extinct at the same time as the dinosaurs, at the end of the Mesozoic era about 65 million years ago. As ammonites died, they sank to the bottom of the sea and were buried, then fossilized in layers of shale. Most ammolite is found in pieces, but occasionally a complete ammonite shell is recovered. These may be up to 3 feet across!

IS AMMOLITE TREATED?

Environmental exposure often damages the surface of ammolite, leaving it unstable and prone to flaking. So, most material is impregnated with clear epoxy before it is polished or cut, to stabilize it. Unlike opal, it is not subject to dehydration and crazing. Ammolite gemstones are usually composites of 2 or more layers, because the ammolite layer is so thin.

 

WHAT FACTORS DETERMINE QUALITY IN AMMOLITE?

There are three features that affect quality:
1) A vivid display of multiple colors;
2) Strong iridescence that can be observed from a wide range of angles;
3) A pleasing pattern of color with minimal gaps, inclusions or fractures.

 

WHAT ARE THE PHYSICAL PROPERTIES OF AMMOLITE? 

Ammolite is CaCO3, a carbonate, with a variable chemical composition. It may include aragonite and calcite, as well as silica, pyrite, and other minerals. Trace elements include aluminum, barium, chromium, copper, iron, magnesium, manganese, strontium, titanium, and vanadium. It has a vitreous luster, a hardness of 3.5 to 4.5, and a specific gravity of 2.6 to 2.9. It is brittle, with distinct cleavage, and has orthorhombic crystallography.

WHAT IS THE HISTORY OF AMMOLITE?

The Blackfoot people of southern Alberta have known about iridescent ammonite fossils for hundreds of years. They called the material "iniskim" (meaning "buffalo stone") and used it as a talisman. In 1908, it was first described in scientific literature in a report by the Canadian Geological Survey. In 1962, it was first exhibited as a gemstone, used as cut gems in jewelry, at a gem show in Alberta. 5 years later, Marcel Charbonneau, who owned a rock shop in Calgary, began to make gemstones out of it, and gave it the name ammolite. It was recognized as a gemstone by the CIBJO in 1981, and in 2004 it became the official gemstone of Alberta. Today it is mined by Korite International and the Aurora Ammolite Mine.

 

 

 

 

 

 

 

 

 

 

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