Ammolite

Organic gems · also called Ammonite shell gem, Korite, Calcentine, Aapoak

Ammolite is the iridescent fossilised nacre of Upper Cretaceous ammonites, cut as an organic gem. Its colour is structural, produced by interference between stacked aragonite platelets, so blue and violet are rarer than red and green. Commercial material comes almost entirely from the Bearpaw Formation of southern Alberta, Canada.

What is ammolite?

Ammolite is the iridescent fossilised shell of ammonites, cut and finished as a gem. The material is nacre, the same aragonite platelet structure that lines a living mollusc shell, preserved through roughly seventy million years of burial with its optical layering intact.

It is an organic gem rather than a mineral variety, and one of very few gems that come from a single geological formation. Nearly all commercial ammolite is taken from the Bearpaw Formation of southern Alberta, Canada, where two Upper Cretaceous ammonite species, Placenticeras meeki and Placenticeras intercalare, carry nacre thick enough to work. The gem layer is usually well under a millimetre deep, which governs almost everything about how the material is cut, backed and sold. As an organic gem it sits alongside amber rather than alongside the crystalline stones.

What gives ammolite its colour?

Nothing absorbs; everything interferes. The colour is structural, produced when light reflects from the many stacked aragonite platelets that make up the nacre and the reflections reinforce or cancel one another. The thickness of the platelets sets the wavelength that survives, and therefore the hue.

Thicker platelet stacks return the long wavelengths, which is why red, orange and green dominate most ammolite. Thinner stacks return the short wavelengths, and blue, indigo and violet are correspondingly scarce and priced accordingly. Because the effect depends on angle as well as thickness, the colour of a given patch changes as the stone is rotated or tilted, and a good piece keeps a strong colour across a wide range of viewing angles rather than flashing once and going dark.

The mineralogy matters here. Aragonite is the metastable form of calcium carbonate and normally recrystallises to calcite over geological time, which destroys the layering and the colour with it. Ammolite exists because burial in the fine marine sediment of the Bearpaw shales happened to preserve the aragonite instead. Fossil ammonites are common worldwide; ammonites that kept their nacre are not.

How is ammolite identified?

By the combination of an opaque carbonate substrate and a broad, angle-dependent iridescence broken into a mosaic of fragments. Aragonite gives a refractive index between about 1.52 and 1.68, with a spot reading near 1.52 in practice, a specific gravity around 2.60 to 2.85, and a hardness of only 3.5 to 4.5.

On a capped stone none of that is directly available, because the refractometer reads the cap rather than the gem: a synthetic spinel cap reads about 1.72, a quartz cap 1.54. That reading is itself diagnostic of an assembly rather than a problem.

The materials it is confused with separate on pattern. Black opal shows play-of-colour from a silica sphere array, with patches that shift and swim rather than sitting in fixed fractured cells. Labradorite and spectrolite show broad sheets of schiller across a translucent feldspar with a hardness of 6 to 6.5. Abalone and paua shell are modern nacre, lighter and without the crackle described below.

The distinguishing feature: dragon skin

Ammolite is characteristically broken into a mosaic of small, tightly fitted plates separated by dark seams, an effect the trade calls dragon skin. It is the record of the shell being compressed and fractured in the ground and then healed by mineral matter filling the cracks, and no other iridescent gem material shows it.

The pattern is a grading axis rather than simply a flaw. Fine crackle across a piece with strong colour is accepted and often valued for its texture; coarse fracturing that isolates the colour into small disconnected islands reduces the piece considerably. The opposite extreme, broad unbroken areas of a single colour, is traded as sheet ammolite and is the scarcest presentation of all.

Is ammolite treated?

Almost always, and openly. Untreated ammolite is a fragile flake of fossil nacre, so the material is stabilised with a clear resin or epoxy that penetrates the fractures and holds the plates together, and it is very commonly assembled: a doublet backs the gem layer with its own shale or with a dark substrate, and a triplet adds a transparent cap of synthetic spinel or quartz on top.

The cap is what makes ammolite wearable at all. It carries the abrasion that the gem layer at 3.5 to 4.5 could not survive, and it raises the apparent depth and saturation of the colour. None of this is concealment when it is disclosed, and the trade convention is to state solid, doublet or triplet plainly. Surface lacquering is also encountered on lower grades. What should be resisted is a triplet sold at the price of a solid stone, since solid ammolite of workable thickness is genuinely uncommon.

Where does ammolite come from?

From the Bearpaw Formation in southern Alberta, and effectively nowhere else in commercial quantity. The productive ground lies along the St Mary River south-east of Lethbridge and around Magrath, where mining has been organised on a commercial basis since the late 1970s.

The deposit is worked as a shallow open operation through marine shale, with the gem-bearing ammonites recovered whole where possible and split to expose the nacre. A deeper zone yields thicker, better-preserved nacre; a shallower weathered zone yields more fragmented material. Iridescent ammonite is known from Madagascar, Morocco, Montana and elsewhere, but the nacre is generally too thin or too poorly preserved to cut, which is why the Alberta material has no real competition. It is a single-source gem in the same practical sense as ametrine.

Where the name comes from

Ammolite is a trade name built on ammonite, the fossil group that supplies it. Ammonite in turn comes through Latin cornu Ammonis, the horn of Ammon, from the ram's horns of the Egyptian god Amun, which the coiled shells were thought to resemble.

The gem name is recent. The material was known to the Blackfoot of the northern plains long before it was cut for jewellery, and it entered the international gem trade only in the twentieth century, with CIBJO recognising it as an organic gemstone in 1981. It is also sold under producer trade names, of which Korite is the most widely encountered; those refer to a company's graded output rather than to a distinct material.

What sizes and forms are available?

Ammolite is sold by the piece, not by the carat, and almost always in freeform shapes. The gem layer follows the curve and the ribbing of a shell, so cutters work with what the fragment gives them rather than imposing a calibrated outline, and matched pairs for earrings are correspondingly difficult.

Small calibrated cabochons and inlay pieces are produced for volume jewellery, but the further a piece is pushed towards a regular shape the more colour is wasted. Large flat expanses of strong, unbroken colour are the genuine rarity, and price rises steeply with clean area. Whole polished ammonites, kept as specimens rather than cut, are a separate market valued on completeness and colour coverage of the shell as an object.

How much is ammolite worth?

Colour first, and specifically how many colours and which ones. A piece showing blue and violet alongside the common red and green sits at the top of the range, because the thin platelet stacks that produce short wavelengths are the scarcest structure in the material.

After that: brightness, the rotational range over which the colour holds rather than dying at one angle, the proportion of the surface actually covered by colour, the fineness of the crackle, the clean area of the piece, and finally whether it is solid, a doublet or a triplet. Producers publish their own AA and AAA style grading scales, which are useful shorthand within one supplier's stock but are not an independent laboratory standard and should not be read as one. Two pieces graded AAA by different sellers are not necessarily comparable.

Is ammolite durable enough to wear?

Only with protection. At 3.5 to 4.5 on the Mohs scale ammolite is softer than everyday household dust, which is quartz at 7, so an exposed surface will abrade and lose its polish through ordinary wear. It also parts readily along the nacre layers.

This is why capped triplets exist, and why ammolite belongs in pendants, earrings and brooches far more comfortably than in rings. A ring set with ammolite should have a hard cap and a bezel that stands proud of the gem surface. Heat and solvents are the other hazards: the stabilising resin and the cement in an assembly can be softened or clouded by acetone, alcohol and household cleaners, and prolonged direct sunlight or dry heat can craze the surface.

Clean it with a barely damp cloth and nothing else. Ultrasonic and steam cleaning will separate an assembled stone and should never be used.

How to buy ammolite without being caught out

Ask, before anything else, whether the piece is solid, a doublet or a triplet, and expect a direct answer. Look at the girdle or the side of the stone under magnification: an assembly shows one or two straight join lines, a colourless cap of even thickness, and often a dark backing that is not the natural matrix.

Then rotate the piece slowly under a single light source rather than looking at one flattering angle. Good ammolite holds its colour across a wide arc; weak material shows one bright position and goes dead a few degrees either side. Check how much of the surface is genuinely covered by colour, since photographs are habitually taken at the one angle where coverage looks complete.

Treat blue and violet claims sceptically until you see them under neutral light, because a strong green can read as blue-green under warm lighting. Take supplier grades as internal shorthand, not certification. Finally, plan the setting before you buy: this is a gem that will look the same in twenty years if it is protected and will not if it is not. If you want us to look for a particular piece, tell us what you are after.

Looking to buy ammolite? See the collection, the rough, or send us a brief.