Guest Author - Susan Dorling

Distinguished by its intensely bright beauty and rarity, ammolite burns with its own unique multi-colored ‘fire’ and no two stones are ever the same. 70-75 million years have passed since this wonder of nature was born, yet it only entered the World market in the 1960’s.

Captivated by the magical beauty of ammolite, enchanted by the fact it is the rarest of all gemstones, and delighted to know the finest iridescent gems are found only in my home country of Canada, I did some research to find you an exceptional source for ammolite jewelry (link is provided at the bottom of the article), and asked our gemology expert, Barbara W. Smigel, PhD. Graduate Gemologist, GIA, for her comments about this remarkable gem.

Barbara tells us, “This gem is the result of the fossilization of extinct, shelled mollusks, called ammonites which are related to today's Nautilus. Although many different ammonite fossils are found in many world locations, only two of the many species found just in a restricted area in Southern Alberta, Canada show the iridescent effect, which has preserved, and enhanced, the thin, tablet-like aragonite crystal layering of the shell.

The iridescence is created by interference between light waves traveling 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 blue and purple. These structural differences help explain why the red material tends to be tougher than the blue or purple. 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.

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 impregnation with a plastic resin, coated with lacquer or made into assembled stones such as doublets or triplets.

A relative newcomer to the gem scene, ammolite began to enter the World market only during the 1960's. Although delicate, and not suitable for some jewelry uses, the gem has many admirers and top quality pieces will fetch high prices. In my opinion, 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 stabilized 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 fine black opal which is at least on order of magnitude more expensive.”

Value

Barbara was asked about the value of this remarkable gemstone and told us, “The value of a piece of ammolite is determined by several different factors: the nature and intensity of the colors, the presence or absence of dead spots or directionality in the iridescence and the degree of fracturing. Due to the fragility of the thinnest layers, specimens with blue or violet color are especially rare and costly. Added to this, is a premium which is accorded to solid pieces which have received no treatment beyond basic resin impregnation. Pieces without stabilization are a possibility for the collector market, but such pieces are all but unusable in jewelry.”

Gemological Properties

Finally what gemologist worth their weight in the rarest of ammolite could resist including the essential ‘gemological properties’ when asked about a gemstone. After all, this is the substance of the gem, the bare bones of the stones. Barbara gave us the following gemological properties of ammolite for our reference: (Watch for our comprehensive Gemstone & Gemological Properties Glossary onsite by the end of August.)

Makeup - fossilized mineralized ammonite shell, usually aragonite: CaC03
Hardness - 3.5
Crystal System - Orthorhombic
Luster - vitreous to resinous
Toughness - red is relatively tough, blue and purple are brittle
Density - usually about 2.76-2.84
Refractive Index - usually 1.52-1.67
Birefringence - 0.135 - 0.145
Cleavage - usually none