also known as Korite or Calcentine, is a rare and beautiful
opal-like organic gem. It is one of the three biogenic
gemstones, the other two being Amber
Pearl. Ammolite is fossilized
Ammonite shell. Ammonites were squid-like sea creatures
that lived about 71 million years ago. Ammonites inhabited a prehistoric, inland subtropical
sea that bordered the Rocky Mountains in the western
United States and Canada. This sea is known today as
the Cretaceous or Western Interior Seaway. As this sea
receded, the Ammonites were buried and crushed by layers
of bentonite sediment. This sediment preserved the shells
consisting of Aragonite,
preventing them from converting to Calcite.
Most fossilized shells from other locations worldwide
have had their Aragonite pseudomorphously replace by
Calcite or Pyrite making the existense of Aragonite
in Ammonite shells particularly rare. Ammonites became
extinct along with dinosaurs at the end of the Mesozoic
outer layer of the Ammonite shell consists of Aragonite and
is what is used to make Ammolite gems. This layer is
usually very thin, up to 1 mm thick, and fragile.
Ammolite gems are typically fabricated into triplets
with quartz or synthetic spinel top layers and a shale
backing. Quartz is a good top with Moh's hardness of
7 but is somewhat brittle. Synthetic Spinel is a better
top with Moh's hardness of 8 and it is more durable.
examples of Ammolite are prized for their beautiful, irridescent,
opal-like play of color. This variety of colors is available
mostly in shades of reds and greens although
all the spectal colors are possible. The irridescence
is due to the microstructure of the Aragonite. The color
of most gems comes from light refraction. The irridescent
colors of Ammolite comes from interference with the
light that rebounds from stacked layers of thin platlets.
Thicker layers produce more reds and greens and thinner
layers produce more blues and violets. Reds and greens
are the more common colors with blues and violets
is available from several sources along the Rocky Mountains.
However, significant deposits of gem-quality Ammolite
are found in the Bearpaw formation that extends from
Alberta to Saskatchewan in Canada and south to Montana
in the USA. Possibly the best grade of gem quality Ammolite
is found on the eastern slopes of the Rockies in southern
Alberta. Most commercial mining of Ammolite is along
the banks of the Lehigh River in an area between the
cities of Summit Hill and Lethbridge.