Ammolite

Ammolite  is a rare and valuable organic gem found primarily on the eastern slopes of the Rockies of the USA y Canada. It is made up of the fossilized shells of ammonites, made up mainly of aragonite, the same mineral from which the pearls pearly.

The chemical composition of ammolite is variable, and in addition to aragonite it may include calcite, silica, pyrite and other minerals. The shell itself may contain some trace elements, including aluminum, barium, chromium, copper, iron, magnesium...

The best samples present an iridescent play of color opaline, with green and red tones; In any case, all colors of the visible spectrum can occur. The iridescence se debe a la microestructura del aragonito: al contrario que muchas otras gemas, cuyos colores provienen de la refracción de la luz, en la ammolita sus colores provienen de la interferencia de la luz que rebota en alguna de sus muchas capas de material apiladas que forman el aragonito. A mayor espesor, los colores son más rojizos y  verdosos, y con menores espesores predominan los azules y los violetas. Los rojos y los verdes son los colores que se presentan más frecuentemente, debido a que los espesores finos son más frágiles y por tanto menos frecuentes. Recién minadas, los colores no son especialmente llamativos; estos aparecen tras un proceso de pulido y otros tratamientos, los cuales muestran la ammolita en todo su esplendor.

The ammolite itself is a thin layer, about 0.5-0.8 mm thick.

Ammolite comes from the fossil shells of ammonites in the shape of a disc Upper Cretaceous, typically the species Placenticeras meeki and Placenticeras intercalare, and to a lesser extent the baculites Baculites compressus.  

The ammonites that gave rise to ammolite inhabited the subtropical internal seas that bordered the Rockies, when said sea disappeared, the ammonites were buried and compressed under layers of benthic sediment. This sediment preserved the aragonite of the shells, preventing its transformation into calcite. Through diagenesis, these shells were impregnated with trace elements present in the sediments, mainly iron and magnesium, which give the ammolite its predominance of green colors.