![]() This sediment preserved the aragonite of their shelled remains, preventing it from converting to calcite. As the seas receded, the ammonites were buried and crushed by layers of bentonite sediment. The ammonites that form ammolite inhabited a prehistoric, inland subtropical sea that bordered the Appalachain Mountains-this area is known today as the Cretaceous or Western Interior Seaway. Ammonites were cephalopods, or squid-like creatures, that thrived in tropical seas until becoming extinct along with the dinosaurs at the end of the Mesozoic era. Map of North America highlighting the shallow inland sea present during the mid-Cretaceous period.Īmmolite 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. Most fossilized shells have had their aragonite pseudomorphously replaced by calcite or pyrite, making the presence of ammolite particularly uncommon. While these shells may be as large as 90 centimeters (35.5 inches) in diameter, the iridescent ammonites (as opposed to the pyritized variety) are typically much smaller. 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. Ammolite mined from deeper deposits may be entirely smooth or with a rippled surface. The cracking results in a tessellated appearance, sometimes described as a " dragon skin" or " stained glass window" pattern. So-called "frost shattering" is common exposed to the elements and compressed by sediments, the thin ammolite tends to crack and flake prolonged exposure to sunlight can also lead to bleaching. Rarely is ammolite without its matrix, which is typically a grey to brown shale, chalky clay, or limestone. The ammolite itself is actually a very thin sheet, ca. ![]() When freshly quarried, these colors are not especially dramatic the material requires polishing and possibly other treatments in order to reveal the colors' full potential. Reds and greens are the most commonly seen colors, owing to the greater fragility of the finer layers responsible for the blues. The thicker the layers, the more reds and greens are produced the thinner the layers, the more blues and violets predominate. The iridescence is due to the microstructure of the aragonite: unlike most other gems, whose colors come from light refraction, the iridescent colour of ammolite comes from interference with the light that rebounds from stacked layers of thin platelets that make up the aragonite. Under ultraviolet light, ammolite may fluoresce a mustard yellow.Īn iridescent opal-like play of colour is shown in fine specimens, mostly in shades of green and red all the spectral colors are possible, however. 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. Its hardness is 4.5–5.5, quite soft for a gemstone, and its specific gravity is 2.60–2.85. The shell itself may contain a number of trace elements, including: aluminium barium chromium copper iron magnesium manganese strontium titanium and vanadium. The chemical composition of ammolite is variable, and aside from aragonite may include calcite, silica, pyrite, or other minerals. The latter is a trade name given to the gemstone by the Alberta-based mining company Korite International, the first and largest commercial producer of ammolite. In 2004 it was designated the official gemstone of the Province of Alberta.Īmmolite is also known as aapoak ( Kainah for "small, crawling stone"), gem ammonite, calcentine, and korite. 1 In 1981, ammolite was given official gemstone status by the World Jewellery Confederation, the same year commercial mining of ammolite began. It is one of the three biogenic gemstones, the other two being amber and pearl. It is made of the fossilized shells of ammonites, which in turn are composed primarily of aragonite, the same mineral that makes up nacreous pearls. Typical of ammolite, reds and greens predominate, and a "dragon skin" pattern is apparent.Īmmolite is a rare and valuable opal-like organic gemstone found primarily along the eastern slopes of the Rocky Mountains of the United States and Canada. An unprocessed sample of ammolite on shale matrix, photographed wet and in natural light to simulate the effects of polishing.
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