Archivo: Ammolite from Placenticeras fossil ammonite, Alberta

Description: Ammolite (5.5 cm across at its widest) - diagenetically altered, iridescent Placenticeras ammonite shell material (nacreous aragonite, CaCO3) from the Upper Cretaceous Bearpaw Formation of the St. Mary River Valley, Alberta, Canada.

Ammolite is biogenic gem material from Alberta, Canada. It has stunningly intense, iridescent rainbow colors. Ammolite is fossil shell material from Placenticeras ammonites. Ammonites are an extinct group of swimming squid-like organisms with planispirally coiled shells (the chambered nautilus in modern oceans is a distant relative of ammonites, but has a similar body plan). Ammonite shells were originally nacreous aragonite (“mother of pearl”) (CaCO3). Geologic studies have shown that ammolite gem material formed from slight diagenetic alteration of the original ammonite nacreous aragonite shell. Diagenesis has significantly intensified and brightened the play of colors.

Ammolite is mined, polished, and treated by resin- or epoxy-impregnation to stabilize it. Very rarely, complete specimens of Placenticeras ammonite shells preserved in ammolite are recovered - such specimens are exceedingly valuable (for example, see figure 2 of Mychaluk et al., 2001).

Name & classification: Placenticeras meeki or Placenticeras intercalare (Animalia, Mollusca, Cephalopoda, Ammonoidea, Ammonitina)

Stratigraphy & age: Bearpaw Formation, Campanian Stage, upper Upper Cretaceous, ~70-75 Ma.

Locality: mine in the St. Mary River Valley west or northwest of Welling and south-southwest of Lethbridge, southern Alberta, southwestern Canada.

Reference cited:

Mychaluk, K.A., A.A. Levinson & R.L. Hall. 2001. Ammolite: iridescent fossilized ammonite from southern Alberta, Canada. Gems & Gemology 37(1): 4-25.
Author: James St. John
Usage Terms: Creative Commons Attribution-Share Alike 3.0
License: CC-BY-SA-3.0
License Link: http://creativecommons.org/licenses/by-sa/3.0/
Attribution Required?: Yes