Key properties
| Chemical formula | CaCO3 (aragonite) with variable calcite, pyrite, silica and trace elements |
| Crystal system | Aragonite (orthorhombic) occurring as stacked sub-microscopic platelets - reads as aggregate |
| Mohs hardness | 3.5-4.5 |
| Toughness | Poor to fair; the iridescent layer is a fraction of a millimetre thick |
| Refractive index | 1.520-1.680 |
| Birefringence | 0.1350-0.1450 |
| Optic character | Aggregate (AGG) |
| Specific gravity | 2.60-2.85 |
| Cleavage | None visible |
| Fracture | Uneven to granular |
| Habit | Coiled spiral fossil shells (Placenticeras and relatives) and their iridescent shell layer (ammolite); specimens up to 60 cm across |
| Lustre | Greasy to dull on rough; vitreous when polished; polish: Vitreous, resinous on capped goods |
| UV fluorescence | Variable; some material shows weak mustard-yellow fluorescence |
| Pleochroism | None |
| Cause of colour | Iridescence from interference of light rebounding through stacked aragonite platelets in the fossilised shell nacre: thicker layers yield reds and greens, thinner layers blues and violets |
| Typical inclusions | Suture (wavy septum) patterns of the ammonite chamber walls; 'dragon-skin' tessellated fracture texture on the iridescent layer; matrix shale backing on raw plates |
| Stability | Fragile: the gem layer is only 0.5-0.8 mm raw and 0.1-0.3 mm after polishing; heat breaks it down and destroys the iridescence; acids attack the carbonate |
| Heat sensitivity | High; loses iridescence with heat |
| Care and cleaning | Mild soap and warm water with a soft cloth; never ultrasonic or steam; a hard brush scratches capped/lacquered material |
| Birthstones | None (official gemstone of the province of Alberta, Canada, designated 2022; official gemstone of the City of Lethbridge, 2007) |
| Availability | Fossil ammonite specimens are common worldwide, but gem-iridescent ammolite is scarce and essentially confined to southern Alberta, where a single producer (Korite) mines most of the world supply |
| Recognition | Ammolite gained official gemstone status with CIBJO in 1981 and a devoted following in Japan and among feng-shui buyers ('seven-colour prosperity stone'); Blackfeet tradition calls it the buffalo stone (iniskim) |
Varieties
Ammonite (Ammolite)
| Colour | iridescent red, orange, gold and green (common) through blue and violet (rare and most valuable) over grey-brown fossil shell |
| Transparency | opaque |
| Clarity type | Opaque |
| Phenomena | Iridescence (interference); 'dragon-skin' tessellated texture; mosaic-like colour patches |
| Appearance | Grey to brown fossil shell flashing broad red, green and gold fire, sometimes blue and violet, in tessellated patches like stained glass; whole spirals show chambered sutures |
| Trade names | ammolite • aapoak (Kainai: 'small, crawling stone') • calcentine • korite (producer trade name) • gem ammonite • iniskim (Blackfeet buffalo stone) |
| Misnomers | ' Alberta opal' (informal; not opal) • 'lumarite' and similar promotional labels |
| Typical sizes | Cabochon and freeform gems from a few millimetres to large tablets; whole ammonite specimens from centimetres to over 60 cm across (non-iridescent or weakly iridescent) |
| Cutting styles | Cabochons, freeforms, flat tablets, doublets and triplets (ammolite on shale backed and capped with optical quartz, synthetic spinel or glass); whole polished shell halves as specimens |
| Key separations | vs labradorite: ammolite is completely opaque with tessellated 'dragon-skin' colour patches; labradorite is translucent at angles from a single cleavage plane flash and much harder (6). vs black opal: opal's play-of-colour comes from within a compact silica body; ammolite's iridescence is a surface shell layer on shale. vs abalone shell: abalone is wavy-banded nacre on a curved shell; ammolite is flat-lying fossil shell. vs lumachella ('fire marble') and Slocum stone: structure and layering differ |
| Sources (localities) | Commercial gem ammolite is effectively one source: the Upper Cretaceous Bearpaw Formation of southern Alberta, Canada (St. Mary River area near Lethbridge, incl. Kainai Nation leases; Korite mines ~90% of supply) • minor material from Saskatchewan and Montana • small occurrences in Utah • plain fossil ammonites found worldwide |
| Availability | Jewellery-grade ammolite scarce; mining seasonal and regulated; specimen ammonites plentiful |
| Market notes | Most finished ammolite is impregnated with epoxy (since 1989) and sold as doublets or triplets - disclose assembly and any caps. Graded AA, A+, A, A- by colour coverage and brightness. Japan is the largest market; feng-shui demand influences prices |
| Care notes | Best suited to pendants, earrings and brooches rather than rings; mild soapy water and soft cloth only; no ultrasonic, steam, acids or harsh heat; check capped edges for chips |
| In short | Ammolite is the iridescent aragonite layer of Alberta's fossil ammonites - red-green fire in tessellated patches, mostly sold as stabilised doublets and triplets, and Alberta's official gemstone. |
| Size (ct) | USD per carat | Tier | As of |
|---|---|---|---|
| 0.50–5.00 | $5.00 – $60.00 | commercial doublets/triplets | 2026-10-26 |
| 5.00–20.00 | $50.00 – $300.00 | fine multicolour (red-green-blue) sheet material | 2026-10-26 |
Retail market survey, per carat unless noted. Ranges describe what commercial to fine stones trade at; exceptional stones and provenance premiums sit above them.
Sources
Data compiled and fact-checked against gemmological literature, October 2026 (JDMIS gem reference). Properties are ranges accepted in gemmological practice; individual stones vary.
See also
Gemstone reference entries: gem #146 · gem #215
Related materials: Corundum · Diamond · Opal · Quartz · Spinel · Topaz
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