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Ammonites were squid-like creatures with sharp, beak-like jaws which had a wide range of shell morphologies (as show in the below image). Ammonites first appeared in the fossil record during the Devonian period some 400 Million years ago, and went extinct at the same time as the dinosaurs at the end of the Mesozoic.
In the mineral and gem worlds the term ammolite is the trade name for the iridescent, nacreous layer of the shell of fossilized ammonite. Ammolite is an opal-like organic gemstone found in various placed in the North America and Madagascar. Ammolite is made of the fossilized shells of ammonites, which in turn are composed primarily of aragonite. The microstructure of the mineral is inherited from the shell. It is one of few biogenic gemstones. Other more commonly known biogenic gemstones include amber and pearl. Ammolite was officially recognised as gemstone species in 1981. The chemical composition of ammolite is highly variable, and apart from aragonite it may also include calcite, silica, pyrite, or other minerals. Additionally – other trace elements have been reported such as aluminium; barium; chromium; copper; iron; magnesium; manganese; strontium; titanium; and vanadium. Ammolite “opalisation” is a term used to express the similarity between the two minerals due to the spectrum of colours it displays. Red and green are fairly common, whilst, blue or purple are rare due to the latter's fragility. Grading of ammolite if largely based on the number and intensity of colours visible, with lower grade material being comparatively dull with less dominant colours.