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Ammolite

Pronounced: AM-uh-lite

Ammolite is a rare, naturally iridescent gemstone formed from the fossilised shells of ancient ammonites. Famous for brilliant flashes of red, green, gold, blue and violet, the finest gem-quality material is associated with the Late Cretaceous Bearpaw Formation of southern Alberta, Canada.

Formula: Predominantly CaCO₃ — calcium carbonate in the form of aragoniteMohs hardness: Approximately 3.5–4System: Orthorhombic — referring to its aragonite component
AmmoliteCW

Meet the crystal

About Ammolite

Ammolite is one of the most unusual gemstones in the world because it began life not as a crystal growing underground, but as the shell of a marine animal. Around 70–75 million years ago, ammonites lived in the inland sea that covered much of what is now western North America. After some of these animals died, their shells became buried within marine sediments. Under exceptional geological conditions, portions of their original aragonite shell were preserved rather than being completely replaced during fossilisation. The resulting material can display extraordinary iridescence, producing brilliant reds, oranges, yellows, greens, blues and violets that shift as the specimen is moved. Commercial gem-quality Ammolite is particularly associated with southern Alberta's Bearpaw Formation and historically with the fossil ammonites Placenticeras meeki and Placenticeras intercalare. The iridescent gem layer is usually very thin and fragile, so jewellery-grade Ammolite is commonly stabilised or constructed as a doublet or triplet for protection. Ammolite was designated the official gemstone of Alberta in 2022, recognising its unusual geological and cultural connection with the province.

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Showing Science & Formation

Scientific passport

The mineral facts

Observable and measurable properties used to describe and identify this mineral.

Chemical formula
Predominantly CaCO₃ — calcium carbonate in the form of aragonite
Mineral class
Organic/biogenic gem material — fossil ammonite shell; predominantly carbonate mineral aragonite
Crystal system
Orthorhombic — referring to its aragonite component
Mohs hardness
Approximately 3.5–4
Lustre
Vitreous to resinous, with strong iridescence
Streak
White for the aragonite component; streak testing is not recommended for gem Ammolite
Specific gravity
Approximately 2.76–2.84 for natural solid Ammolite; values can vary because of shell structure, asso
Transparency
Generally opaque; very thin shell layers may be translucent

Common colours

Red, orange, yellow, gold, green, blue and violet, often with several colours visible within the same specimen

Cleavage and fracture

No readily observed true cleavage in gem Ammolite; strong parting can occur along the thin shell layers. The material is brittle and naturally prone to fracturing and delamination. The GIA study of Alberta Ammolite measured specific gravities mostly around 2.76–2.84 and describes the material as predominantly soft aragonite with pronounced sheet-like parting.

From the Earth

How it forms

The geological processes that create this crystal over time.

Formation

Its geological story

Ammolite begins with the shell of an ammonite, an extinct marine cephalopod. During the Late Cretaceous, ammonites lived in the sea that covered what is now southern Alberta. After death, some shells sank into seafloor sediment and became buried within mud that later formed the Bearpaw Formation. Normally, the original shell material of a fossil can dissolve or be replaced by other minerals. Under the unusual burial conditions present in parts of southern Alberta, portions of the original aragonitic shell were preserved. Over millions of years, burial, pressure and mineralisation preserved the extremely fine layered shell structure responsible for Ammolite's colour. When light enters these microscopic aragonite layers, different wavelengths interfere with one another, creating the brilliant iridescence seen at the surface. The Bearpaw Formation itself consists largely of marine shale and siltstone deposited within an ancient inland sea.

A closer look

Scientific notes

Ammolite is unusual because it is simultaneously a fossil-derived material and a gemstone rather than an individual mineral species. Its vivid colours are not primarily caused by coloured pigments. Instead, its iridescence is a structural optical effect. Light interacts with extremely thin layers of aragonite within the fossil shell, causing interference that allows certain wavelengths to be reflected more strongly than others. The thickness and condition of these layers influence the colours observed. Red and green are relatively common, while strong blue and violet can be less common because they require finer preserved structures. Classic Canadian Ammolite comes from certain horizons of the Bearpaw Formation and is associated particularly with Placenticeras meeki and P. intercalare. The original aragonite shell layer can be only a fraction of a millimetre to a few millimetres thick after preparation, explaining why the material often needs reinforcement before use in jewellery.

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