AmmoliteWhile not the only fossil that is used as a gem (five pointed sea urchins spring to mind), t

AmmoliteWhile not the only fossil that is used as a gem (five pointed sea urchins spring to mind), the opal like sheening ammonite shell from the Rocky mountains of Canada and Utah is certainly the most beautiful. I have frequently seen it mislabelled as opal, or opalised ammonite, though it is actually fossilised nacre (0.3-0.8mm thick), similar to that of modern nautilus shells (living relatives of ammonites) or pearls, rather than any kind of replacement with silica.Dating from the late Cretaceous, the usually red or green sheen on the nacre is due to a phenomenon called thin film interference, something you all see in the sheen of petrol puddles on water. The thin layers of aragonite forming the shell reflect the light differently, creating waves of interference whose colour depends on the thickness of the plates, which are of similar size to the wavelengths of light. Thicker layers produce reds and greens, and thinner ones the rarer blue and violets. Southern Alberta in the Cretaceous was a shallow sub-tropical inland sea, and the dead shells were preserved from decay in volcanic ash, that gradually altered into protective clays.The colours are enhanced by polishing, and being a fragile material (Mohs hardness 4.5), thin slices of the shell are often used to make doublets or triplets (much like with opal), where layers of harder material such as dyed black onyx or quartz are used to protect the gem itself. As with opal, the wider the spread of playing colours as the stone is moved in bright light, the greater the value. Many different fracture patterns exist, caused by tectonic compression, with a variety of exotic names. Value is also negatively affected by the presence of cracks. It has also been stabilised with resins when exceptionally fragile. All such treatments are fully acceptable, as long as they are clearly declared.The main commercial mine is in Southern Alberta, mining mostly fragments from the Bearpaw shale and finding about 50 intact specimens a year, whose export from Canada needs a permit. Nacreous ammonites occur worldwide, but the colours from here are the best, as is the suitability of jewellery use. The same care to avoid exposure to shock and caustic substances applies to ammolite as to pearls, unless it is protected in a triplet gem. Some only have one colour, others the full spread.…LozImage credit: The Boudreaux collectionhttp://www.gemsociety.org/info/gems/ammolite.htmhttp://www.rubybluejewelry.com/main_links/ammolite-history_mineralogy_and_lore.htmhttp://www.espyjewelry.com/index.cfm/fa/subcategories.main/parentcat/6442/subcatid/29574 -- source link

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