AMMOLITE: LOVE, GREED AND GLORY

Ammolite from the Bearpaw Formation

Ammolite is an opal-like organic gemstone found primarily along the eastern slopes of the Rocky Mountains of North America. 

The mining of ammolite is a serious business. I happened upon a locality while in search of fossilized oysters along the St. Mary's River in Alberta. It was one of the few times that I have ever been shot at. 

They sunk the wee boat I was using as a raft to haul my finds but I will give them credit for firing warning shots and not actually trying to hit me. With that, I can safely say that ammolite inspires strong emotions amongst fossil collectors — love, greed and glory. 

It is made of the fossilized shells of ammonites, which in turn are composed primarily of aragonite, the same mineral contained in shell nacre, with a microstructure inherited from the shell. It is one of few biogenic gemstones along with amber and pearl.

The chemical composition of ammolite is variable. Aside from aragonite, it may include a mix of calcite, silica, pyrite or wee bits and pieces of other minerals. The shell itself may contain a number of trace elements based on the chemical composition of the original sediments where it was fossilized and chemical goodies carried in from groundwater. Most anything can be found in the mix, but primarily we see aluminium, barium, chromium, copper, iron, magnesium, manganese, strontium, titanium, and vanadium. 

Its crystallography is orthorhombic, a seven-sided crystal system. Its hardness is 3.5–4.5, and its specific gravity is 2.60–2.85. The refractive index of Canadian material (as measured via sodium light, 589.3 nm) is as follows: α 1.522; β 1.672–1.673; γ 1.676–1.679; biaxial negative. Under ultraviolet light, ammolite may fluoresce a mustard yellow.

Ammolite comes from the fossil shells of the Upper Cretaceous disk-shaped ammonites Placenticeras meeki and Placenticeras intercalare, and to a lesser degree, the cylindrical baculite, Baculites compressus. The ammonites that form our Alberta ammolite inhabited a prehistoric, inland subtropical sea that bordered the Rocky Mountains — the Cretaceous Western Interior Seaway. 

As the cephalopods died, they sank to the bottom and were buried by layers of bentonitic mud that eventually became shale. Many gem-quality ammonites are found within siderite concretions. These sediments preserved the aragonite of the shells, preventing them from converting to calcite.

Ammolite: Colourful Microstructure of Aragonite

An iridescent opal-like play of colour is shown in fine specimens in shades of yellow, orange, red, green and gold. 

The iridescence is due to the microstructure of the aragonite: unlike most other gems, whose colours come from light absorption, the iridescent colour of ammolite comes from interference with the light that rebounds from stacked layers of thin platelets that make up the aragonite. 

The thicker the layers, the more reds and greens are produced; the thinner the layers, the more blues and violets predominate. Reds and greens are the most commonly seen colours, owing to the greater fragility of the finer layers responsible for the blues. 

When freshly quarried, these colours are not especially dramatic; the material requires polishing and other treatments to reveal the specimen's full-colour potential.

Ammolite itself is quite thin, generally 0.5–0.8 millimetres (0.02–0.03 inches) thick. This thin coating covers a matrix typically made up of grey to brown shale, chalky clay, or limestone. Truly, when you find these ammonites in the field, they do not look like much. They are perhaps a nice shape but often matte grey and unappealing until prepared.  

Frost shattering of these specimens is common. If left exposed to the elements the thin ammolite tends to crack and flake. Prolonged exposure to sunlight can also lead to bleaching of the generally intense colouration. The cracking results in a tessellated appearance, sometimes described as a "dragon skin" or referred to as a stained glass window pattern. 

Ammolite mined from deeper deposits may be entirely smooth or with a rippled surface. Occasionally a complete ammonite shell is recovered with its structure well-preserved: fine, convoluted lines delineate the shell chambers, and the overall shape is suggestive of a nautilus. While these shells may be as large as 90 centimetres (35.5 inches) in diameter, the iridescent ammonites — as opposed to the pyritized variety — are typically much smaller. Most fossilized shells have had their aragonite pseudomorphously replaced by calcite or pyrite, making the presence of ammolite particularly uncommon.

In 1981, ammolite was given official gemstone status by the World Jewellery Confederation (CIBJO), the same year commercial mining of ammolite began. It was designated the official gemstone of the City of Lethbridge, Alberta in 2007.

Ammolite is also known as aapoak — Kainah for "small, crawling stone" — gem ammonite, calcentine, and Korite. The latter is a Trade name given to the gemstone by the Alberta-based mining company Korite. Roughly half of all ammolite deposits are contained within the Kainah (Kainaiwa) reserve, and its inhabitants play a major role in ammolite mining. Marcel Charbonneau and his business partner Mike Berisoff were the first to create commercial doublets of the gem in 1967. They went on to form Ammolite Minerals Ltd.