Other ammonite species subject to different fossilization processes have left casts, impressions and petrified remains with and without pyritization in many other places around the world.
[Fossil Ammonites (not Ammolite): petrified animal showing the interior chambers, pyritized ammonite fossil showing some of exterior shell detail]
A relative newcomer to the gem scene, ammolite began to enter the world market only during the 1960's. The iridescence is created by interference between light waves travelling through layers of thin, tablet-like aragonite crystals. The thickness of the individual crystals determines the color seen, with thicker ones giving red and the thinnest ones creating purple. These structural differences help explain why the red material tends to be tougher and more common than the blue or purple. Consequently the reds cost less than blues, all else being equal.
The iridescent layer of these fossils is generally very thin, no more than 8 mm before polishing and as thin as .1 to 3 mm afterward. For this reason and because of its low hardness (3.5) most ammolite is either stabilized by vacuum assited impregnation with a plastic resin, coated with lacquer, or made into assembled stones such as doublets or triplets. With such treatment, ammolite is suitable for jewelry use.
In order of rarity the colors are: red, gold, green, blue-green, blue, violet and purple. Patterning varies, with some pieces showing color sections in big blocks, others with finely veined netting and still other pieces showing a single solid color.
Ammolite is a spectacular, under-used gem material with tremendous potential for greater use by designers as long as the settings are protective or assembled or coated pieces are used. Prices are relatively high for a cabochon material, but with its vivid colors and distinctive patterns, ammolite holds up very well in an aesthetic comparison with black opal which is at least an order of magnitude more expensive.
Added to this, is a premium which is accorded
to solid pieces (non-assembled) which have received no treatment
beyond basic resin impregnation. Pieces without stabilization are a
possibility for the collector market, but use of such pieces for
jewelry is way too risky. Although little has been written of a
specific nature about ammolite prices, it is reasonable to expect
premium prices for large sizes, rare colors, attractive patterns and
freedom from blemishes.