Ammolite 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. 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. It is one of several biogenic gemstones; others include amber and pearl.1 In 1981, ammolite was given official gemstone status by the World Jewellery Confederation, the same year commercial mining of ammolite began. It was designated the official gemstone of the Province of Alberta in 2004 and the official gemstone of the City of Lethbridge in 2007.[2][3]

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 International, the first and largest commercial producer of ammolite.

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Properties

The chemical composition of ammolite is variable, and aside from aragonite may include calcite, silica, pyrite, or other minerals. The shell itself may contain a number of trace elements, including: aluminium; barium; chromium; copper; iron; magnesium; manganese; strontium; titanium; and vanadium. Its crystallography is orthorhombic. Its hardness is 4.5–5.5, quite soft for a gemstone, 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.

An iridescent opal-like play of color is shown in fine specimens, mostly in shades of green and red; all the spectral colors are possible, however. The iridescence is due to the microstructure of the aragonite: unlike most other gems, whose colors come from light absorption, the iridescent color 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 colors, owing to the greater fragility of the finer layers responsible for the blues. 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.

The ammolite itself is actually a very thin sheet, ca. 0.5–0.8 millimeters (0.02–0.03 inches) in thickness. Rarely is ammolite without its matrix, which is typically a grey to brown shale, chalky clay, or limestone. 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. The cracking results in a tessellated appearance, sometimes described as a "dragon skin" or "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 centimeters (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.