AMMOLITE
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 from the nacreous aragonite.
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.
Ammolite (above & below) (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.
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.