The Winter Olympics showcase athletic prowess, but behind every sport lies a surprising amount of science. From specialized skating blades to high-tech ski gear, athletes rely on precise equipment. But few realize that even curling, with its seemingly simple stones, depends on a remarkably specific geological phenomenon. For over a century, the world’s best curling stones have come from just two places: Ailsa Craig, a small Scottish island, and the Trefor granite quarry in Wales. This isn’t arbitrary; the unique properties of these rocks make them uniquely suited for the sport.
Why These Rocks?
Curling stones aren’t just heavy, polished granite. They consist of two critical parts: the running surface, which skids across the ice, and the striking surface, which collides with opponent stones. Each surface demands different physical characteristics to perform optimally. The Ailsa Craig and Trefor quarries provide rocks that meet these criteria in ways no other sources have consistently matched.
These rocks are granitoids, formed from cooled magma, but their age and formation matter. Ailsa Craig’s stones are relatively young, around 60 million years old, formed during the Atlantic Ocean’s creation. Trefor’s are slightly older, 400–500 million years old, born from ancient mountain-building events. This relative youth minimizes internal stresses, making the rocks more durable under repeated impacts.
Debunking Myths and Unveiling the Science
For decades, the prevailing belief was that these stones were ideal because they contained minimal quartz, a brittle mineral prone to fracturing. Mineralogist Derek Leung, formerly of Team Hong Kong, challenged this assumption with recent analyses. Surprisingly, all four rock types (Ailsa Craig common green and blue hone, Trefor blue and red) do contain quartz. However, Leung found minimal fracturing in any of them, likely due to their geological age.
The key lies in grain size. Ailsa Craig blue hone, used for the running surface, boasts small, uniform grains. This prevents chunks from breaking off during sliding, maintaining predictable behavior. It’s also nonporous, reducing ice water intrusion that could cause cracks. The striking surface, however, requires larger grain size variations to resist damage from collisions. Ailsa Craig common green, blue Trefor, and red Trefor provide this necessary toughness.
The Future of Curling Stone Geology
Currently, the 2026 Winter Olympics will use Ailsa Craig common green for the main stone body, with blue hone inserts for the running surface. But the Ailsa Craig quarry is no longer active due to environmental concerns, raising questions about long-term sustainability.
While other locations could theoretically produce suitable stones, past attempts have failed. One Canadian experiment using anorthosite from Northern Ontario resulted in quickly chipping rocks. Leung believes that by identifying formation environments similar to Ailsa Craig—perhaps in Nova Scotia, on the opposite side of the Atlantic rift—it might be possible to discover new sources. Further testing, including actual curling trials, would be crucial.
The future of this precision sport may depend on finding the next geological sweet spot.
The story of curling stones underscores how even niche athletic equipment relies on complex natural processes. The combination of age, mineral composition, and grain structure makes Ailsa Craig and Trefor granite uniquely suited for the game. As supplies dwindle, the search for new sources will require a deeper understanding of the geology that makes curling possible.
