Magrath — The Alberta Town at the Heart of the Ammolite Trade
Magrath — The Alberta Town at the Heart of the Ammolite Trade
Southern Alberta locality where the Bearpaw Formation yields the world's only commercial source of gem ammolite
Magrath is a small town in southern Alberta, Canada, sitting roughly thirty kilometres south of Lethbridge in the Oldman River basin. The town itself is unremarkable to the gem trade, but the surrounding sediments of the Bearpaw Formation host the world's only commercially significant source of gem-quality ammolite, the iridescent fossilised aragonite of Late Cretaceous ammonite shells. Korite International, the principal operator, has worked deposits near Magrath since the late twentieth century, and the locality name is now effectively synonymous with the species in trade catalogues and gemmological literature.
Geological setting
The Bearpaw Formation is a marine shale unit deposited approximately seventy to seventy-four million years ago in the Western Interior Seaway, the shallow inland sea that bisected North America during the Late Cretaceous. The shale preserves the shells of ammonite cephalopods — chiefly Placenticeras meeki and Placenticeras intercalare, with subordinate Baculites — whose original aragonitic shell material has, in this particular setting, survived diagenesis intact rather than recrystallising to calcite or dissolving away.
The preservation owes something to the fine-grained, low-permeability character of the host shale, which sealed the shells from oxygenated groundwater after burial. The aragonite layers, originally laid down by the living animal as a stack of platy crystals roughly four hundred to seven hundred nanometres thick, act as a natural diffraction grating. Light reflected from successive layers interferes constructively or destructively as a function of layer thickness, producing the spectral play-of-colour that defines gem ammolite.
The Korite operation
Commercial mining at Magrath began in the late 1970s and was placed on a stable industrial footing by Korite International, which holds the principal mining leases on the Kainai (Blood Tribe) reserve and on adjacent private and provincial lands. The operation is shallow open-pit work in the Bearpaw shale, with the gem material recovered, washed, sorted, and stabilised before being supplied to cutting partners worldwide.
Ammolite as recovered is fragile — the aragonite layers are thin, brittle, and prone to splitting along the original shell laminae. Most commercial material is therefore stabilised with epoxy or a clear acrylic resin and assembled into doublets or triplets, with a dark backing to enhance contrast and a crystal or quartz cap to protect the surface. Solid ammolite cabochons exist in the highest grades but are uncommon and command a premium.
Trade significance
Ammolite was recognised as an official gemstone by the World Jewellery Confederation (CIBJO) in 1981 and is the only organic gem unique to a single geographic region. The vast majority of supply originates within a narrow corridor of southern Alberta centred on Magrath and the neighbouring Kainai lands. This single-source character is rare in the gem trade and gives ammolite a tightly defined provenance story, which the producing companies have built into their marketing.
In quality terms, ammolite is graded on the breadth of spectral colour visible, the brightness and contrast of the play, the rotational range over which colour remains visible, and the cleanliness of the surface pattern. The most prized stones show all spectral colours including the rarer reds and violets, with strong saturation and a broad rotational range. Single-colour pieces — typically green or red dominant — are common and trade at lower prices.
Identification and care
Ammolite is unmistakable under microscopy because the iridescence sits on visibly stacked aragonite plates rather than within the body of the stone. Refractive indices are approximately 1.52 to 1.68, with a reading complicated by the doublet or triplet construction of most commercial pieces. Hardness of unstabilised aragonite is around 4.5, which makes ammolite a soft stone unsuitable for unprotected ring use; setting in pendants, earrings, and protected ring designs is standard. Stabilised and capped pieces tolerate normal wear, but ultrasonic and steam cleaning should be avoided, as both can attack the resin layer or separate doublet components.
In Gems & Gemology
The gemmology of Magrath ammolite has been documented in several articles in Gems & Gemology, principally by Mychaluk and co-authors, covering the geological setting, the optical mechanism of play-of-colour, the principal cutting and stabilisation methods, and field reports on the producing leases. These remain the standard technical references for ammolite and are recommended reading for any dealer or appraiser handling the material.