Why curling stones come from only two places on Earth

The rocks used in the Olympic sport of curling come from an island in Scotland and a quarry in Wales. What makes them so special?

By Andrea Thompson edited by Seth Fletcher

From skating to curling, the exciting sports of the Winter Olympics are backed by a wealth of scientific data. Follow our coverage here to find out more.

Athletes often have specialized equipment or clothing that allows them to run, swim, skate or ski at their best, but curling takes things to another level. Curling stones, as the round stones weighing about 40 pounds are called, come from only two places on the planet: a small island in Scotland called Ailsa Craig and the Trefor granite quarry in Wales.

But what makes the rocks in these locations particularly suited to sliding across a rectangular ice sheet toward target after target? And are these really the only places to find suitable stones?

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To find out, Scientific American spoke with Derek Leung of the University of Regina in Saskatchewan, a mineralogist and curling enthusiast who competed for the Hong Kong team. Leung married his two interests by performing the first mineralogical analyzes of curling rocks since 1890. “It had been over a hundred years since we last looked at them,” he says, so he wanted to see what modern science could tell us.

But before we get into that, let’s review the two main parts of a curling stone: the running surface and the striking surface. The first is a ring at the bottom of the rock that slides across the ice, and the second is a band around the sides of the rock that collides with other rocks (ideally to knock an opponent’s rock off the target or to bring your team’s rock closer).

Each surface needs specific properties to do its job. This is where Ailsa Craig and Trefor Quarry come into play. The first has been used for curling stones since the beginning of the 19th century; the latter since curling gained popularity after World War II.

Each location has two types of rock used for curling stones: the common green Ailsa Craig and the blue Ailsa Craig stone, as well as the blue Trefor and the red Trefor. All four types are granitoids, which are igneous rocks, meaning they form when magma or lava cools and crystallizes. The Ailsa Craig rocks were formed about 60 million years ago when magma sank into a relatively shallow layer of the Earth’s crust during rifting that formed the Atlantic Ocean. The Trefors formed around 400 to 500 million years ago during a mountain-building event called the Caledonian Orogeny. Both are, geologically speaking, quite young. “Having a young rock is probably a good thing because it means it’s less likely to have been stressed by different tectonic events” before being subjected to the stresses of curling, Leung says.

Common wisdom holds that the rocks from these two sources were ideal because they contained very little quartz, a brittle silica-based mineral that would be less than ideal for expensive stones that were constantly colliding. (Curling stones cost about $600 each and are typically used for 50 to 70 years.) But Leung discovered that all four rocks actually contain quartz. Yet under the microscope, “I found almost no fractures,” he says, probably because of their young age.

Ailsa Craig Blue Diamond is commonly used for the running surface; makers cut a circle from the bottom of the main rock and insert a disc of bluestone. Leung discovered that bluestone had small and fairly uniform grain sizes. This is ideal for decades of sliding on ice, because larger mineral grains are more likely to be stripped away by the ice, leaving holes in the surface that could cause unpredictable behavior. Bluestone is also relatively non-porous, meaning water from ice is less likely to penetrate and cause fractures.

On the other hand, for the striking surface, “you need to have larger differences in grain size,” explains Leung, because “this avoids certain types of damage” when the stones collide. Ailsa Craig Common Green, Trefor Blue and Trefor Red are all good for this, which is why they are used for the main rock; the striking surface is created by cutting the main rock. The rocks at the Milan Cortina 2026 Winter Olympics will be made in common Ailsa Craig green and will use bluestone for the racing surface.

In principle, there is no reason why rock from elsewhere should not work for curling. After all, when this 1890 study was conducted, curlers were using stones from all over Scotland, the birthplace of the sport. Ailsa Craig and Trefor Granite Quarry have probably become go-to sources over time thanks to a combination of their characteristics of performance, tradition and standardization. And blasting is no longer allowed at Ailsa Craig, a now-uninhabited bird sanctuary, so another source would help keep curling clubs stocked in the future.

This is something Leung hopes to work on. An effort in Canada in the 1950s failed because the very black igneous rock called anorthosite, quarried in northern Ontario, began to flake soon after use. But with better knowledge of which rock works best, it might be possible to find a new source. “We could be looking for rocks formed in a similar environment to Ailsa Craig,” says Leung, perhaps in Nova Scotia, which is on the other side of the Atlantic rifting that gave rise to the granitoids found at Ailsa Craig.

He hopes to one day work with quarries to obtain samples for analysis. If he finds any potential candidates, he’d love to turn them into curling stones and try throwing them on the ice to see what happens.