Ammolite

Ammolite is the iridescent fossilized shell of ammonites - extinct cephalopods related to nautilus that lived 71-75 million years ago during the Late Cretaceous period. The gem material comes almost exclusively from the Bearpaw Formation in southern Alberta, Canada, where ammonite shells were preserved in marine sediment under specific conditions that maintained the aragonite shell structure.

The rainbow iridescence is caused by thin-film interference in the layered aragonite structure of the fossil shell. Light reflecting between microscopic aragonite layers creates spectral colors similar to the mechanism in opal, but more vivid and with a more metallic quality. The specific colors depend on the thickness of the aragonite layers - thinner layers produce blue and violet, thicker layers produce red and green.

Ammolite was granted official gemstone status in 1981 by CIBJO (the World Jewellery Confederation). The primary mining company, Korite International, holds claims along the St. Mary River in Alberta. The gem layer is typically very thin (0.5-8mm), so most ammolite jewelry uses doublets or triplets with a protective cap.

Ammolite is identified by its distinctive iridescent play of color on what is clearly fossilized shell material. The colors shift and move as the stone is rotated, similar to opal but with a more metallic, directional quality. The underlying fossil structure (shell growth lines) may be visible.

Distinguish from labradorite (silicate mineral, different iridescence mechanism), opal (amorphous silica, different structure), and abalone shell (modern, not fossilized, different pattern). Ammolite's iridescence is more vivid and directional than most other iridescent natural materials.