Monday, December 23, 2013

Ammolite lifted in Smash and Grab

A thief smashed the front window of a downtown Vancouver store and made off with a rare Canadian gemstone worth half a million dollars, police said. The 11-inch ammolite gemstone was stolen from Gastown's Rocks and Gems Canada at 313 Water St. around 4 a.m. PT Friday. The piece is made of fossilized shells of ammonites, the same mineral that makes up pearls.

Ammolite is the official gemstone of Alberta and is considered a Canadian national treasure. Police said an application to the Canadian government must therefore be made for it to be removed from the country.

Ammolite is an opal-like organic gemstone found primarily along the eastern slopes of the Rocky Mountains of North America. 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 few biogenic gemstones; others include amber, pearl and coral.
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.
The ammonites that form ammolite inhabited a prehistoric, inland subtropical sea that bordered the Rocky Mountains—this area is known today as the Cretaceous or Western Interior Seaway. As the seas receded, the ammonites were buried by layers of bentonite sediment. This sediment preserved the aragonite of their shelled remains, preventing it from converting to calcite.