During that great 300-million-year flowering of marine life called the Paleozoic era, a squid-like animal known as the ammonite thrived in every ocean. This mollusk made its home in giant coiled shells similar to those of the chambered nautilus that we see today. Just like the dinosaurs that emerged later in this geologic age that began 500 million years ago, the ammonite eventually suffered a species wipeout.
Today, ammonites are found fragilely fossilized in various shells all over the world. Usually what remains are dull-colored agatized shell imprints in a host material. This imprint shows the increasingly larger quarters the growing animal built for itself in spiral fashion, walling off each new dwelling space from the previous one. Occasionally, when these aragonite shells molds are lined with nacre, they exhibit a pearly iridescence. The most iridescent and best preserved of these fossils go to museums, but some are used as pendants and brooches.
Now wouldn’t you think that the nacre-coated mollusk remains found on every former sea floor from South Dakota to Tibet would have become prime gem material sometime during the 200 million years since the Paleozoic era? After all, dinosaur bones are found in a gemmy petrified state. Why not gemmified ammonite?
Well, there is at least one place where ammonite shell metamorphosed into a bona fide gem material. Called, appropriately enough, “ammolite,” it is retrieved from shale found 20 feet or so below the ground at various spots throughout Alberta, Canada. The richest of these sites, located at Lethbridge, Alberta, was discovered in 1979 and ever since has been owned and operated by Korite Minerals Ltd., headquarters in Calgary, Canada. Since the Lethbridge deposit produced at least 90% of the world’s ammolite, Korite could be considered a De Beers for this gem, albeit a thimble-sized version that controls the market by default rather than design. Even so, the Canadian outfit has learned a lesson or two from the London-based company. The most important one can be summed up in just five letters: J-A-P-A-N.
From Shell to Shellac
Unlike most ammonites, which died and left aragonite impressions of their outer casings in the sediment at the bottom of the ocean, ammolite was transformed from shell to gem altogether. Korite’s Pierre Pare explains how:
“The empty shell fell to the bottom of the ocean. Over millions of years, a concretion formed around the shell and it became a nucleus that was sealed off from the destructive effects of water and oxygen. As this nodule grew in size, it acted like a pressure cooker to re-mineralize and re-crystallize the ammonite.”
In a superb essay on ammolite published in the January 1986 Lapidary Journal, gemologist Fred Pough describes the gem’s creation as a 70-million-year makeover, during which “the deeply buried [ammonite shell] fragments have been squeezed, compacted and marinated” in a mineral-rich mudpack. Evidently, Alberta’s mudpack formula was unique because ammonites turned to ammolite aren’t as yet found elsewhere. Too bad. This shell re-born as rough is glazed with a mother-of-pearl finish so superior-to-any-later-day variety Pough calls it “grandmother-of-pearl.”
Mining of ammolite is really long-odds excavation. Every year, Korite digs around 20 feet beneath the gravel at its Lethbridge site and extracts tens of thousands of nodules from the shale that it hopes contain cores of gem-quality material. But only 1% do. While these cores are, in effect, nuclei, don’t think of them the way you would the bead-nuclei of pearls. Instead, these nuclei have formed as thin layers inside hardened sediment. Because the original ammonite shell has disintegrated and transformed, ammolite shouldn’t be viewed as a fossil but, rather, as a reconstituted mineral.
Opal of the Sea
At first glance, ammolite could be confused with opal. But while it displays the same bright spectral colors as fine black opal, it does so in a far different way. Ammolite is, in essence, a laminate of crushed and re-crystallized aragonite on a backing of sediment that has hardened over eons into what gemologist Edouard Guilbain called a “dark jasper-like rock.” When stones are cut, this aragonite layer is sawn off and polished. Since the aragonite usually re-crystallizes in discontinuous form, you see a scaly, random arrangement of sequin-like patches bordered by dark-brown host material.
The difference from opal becomes apparent when you turn ammolites in your hand. Although more brilliant and highly iridescent than opal, “the spectral succession is far less dramatic,” writes Pough. In short, ammolite colors shift far less than those of opal. Red, yellow and green seem the most common colors, blue and violet the least.
Ammolite’s strong resemblance to black opal has endeared the stone to Japan, a country where demand is now so strong for these two phenomenon stones that its market has the suction power of a black hole for both gems. Indeed, since Korite started marketing ammolite to Japan in 1986, mushrooming demand there has come to account for 65% of the company’s sales—leaving America, formerly the leading market for this gem, a far distant second.
Booming exports to Japan bet a two-edged sword. Short-term, they have meant the best bottom line in Korite’s history. But long-term, they could mean much less of a life span for Korite’s Lethbridge deposit. Until 1986, Korite produced around 25,000 finished stones a year. Since then, gluttonous Japanese demand has forced it to more than double production to 60,000 stones last year. That requires excavation of 1 1/4 acres a year instead of the once acre a year the company formerly unearthed at its 30-acre quarry.
If Korite is to sustain production well into the next century, it will have to cultivate jeweler acceptance for its less lively stones, says Pare. Presently, the company produces three grades of material, which it classifies AA, A and B. About 95% of these stones are offered to doublet form with synthetic spinel tops, essential because otherwise the ammolite is too thin to sell. Besides, with a hardness of 4½, the spinel cap acts as a protective barrier. Ammolite doublets are currently priced by the grade at $45, $35 and $17 per piece for a popular 11mm x 9mm cabochon size—a bargain for so rare a gem.
Even rarer are solid ammonites, around 4,000 of which are cut every year, usually in free-form style and weighing on average, 20 carats. Wholesaling for $18, $11 and $5 per carat, again according to grade, these stones are not for casual wear. Consequently, buyers should be given written care and handling instructions.
Please note: this profile was originally published in 1992 in Modern Jeweler’s ‘Gem Profiles/2: The Second 60’, written by David Federman with photographs by Tino Hammid.
The 21mm x 29.5mm ammolite shown in the header image was courtesy of Korite Minerals Ltd., Calgary, Alberta.
