The Geology Behind Olympic Curling Stones

Curling stones, weighing roughly 40 pounds each, are essential to the sport of curling, particularly at the Winter Olympics. These stones originate exclusively from two locations: Ailsa Craig, a volcanic island in Scotland, and the Trefor granite quarry in Wales. The geological properties of the stones from these sites make them uniquely suited for sliding across ice towards a target. Derek Leung, a mineralogist from the University of Regina in Saskatchewan, has conducted the first mineralogical analyses of curling rocks since 1890, aiming to uncover the science behind their effectiveness.

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Each curling stone consists of two main parts: the running surface, which skids across the ice, and the striking surface, which collides with other stones. Ailsa Craig has been a source for curling stones since the early 19th century, while Trefor has supplied stones since the post-World War II era. The stones from both locations are granitoids, formed from magma cooling and crystallizing. Ailsa Craig's rocks formed approximately 60 million years ago, while Trefor's originated 400 to 500 million years ago during the Caledonian Orogeny.

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Traditionally, it was believed that the ideal curling stones contained minimal quartz, a brittle mineral. However, Leung's research revealed that all four types of stones—Ailsa Craig common green, Ailsa Craig blue hone, blue Trefor, and red Trefor—contain quartz but exhibit minimal fractures, likely due to their relatively young geological age. The blue hone, often used for the running surface, has small, uniform grain sizes, which is advantageous for consistent sliding over decades.

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The striking surface requires larger differences in grain size to prevent damage during collisions. Ailsa Craig common green, blue Trefor, and red Trefor stones are effective for this purpose. For the upcoming 2026 Milano Cortina Winter Olympics, the stones will utilize Ailsa Craig common green for the main body and blue hone for the running surface.

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While Ailsa Craig and Trefor have become the standard sources for curling stones, Leung suggests that other rocks could potentially serve this purpose. Historical practices included using stones from various locations in Scotland, and there is no inherent reason that stones from other regions could not be suitable. However, the current restrictions on blasting at Ailsa Craig, now a bird sanctuary, highlight the need for alternative sources to ensure a steady supply for curling clubs.

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Leung aims to explore new potential sources for curling stones, particularly in regions like Nova Scotia, which may have geological characteristics similar to Ailsa Craig. Past attempts to source stones from Northern Ontario were unsuccessful due to rapid wear. With advancements in geological understanding, Leung hopes to identify and test new rock types for their suitability in curling, potentially expanding the options available for this traditional sport.

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• “It’s been more than a hundred years since we had looked at them,” — Derek Leung, Mineralogist

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• “Having a young rock is probably a good thing because it means that it's less likely to have incurred stresses related to different tectonic events” — Derek Leung, Mineralogist

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• “We could look for rocks that are formed in a similar environment as Ailsa Craig,” — Derek Leung, Mineralogist

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