Why do all Olympic curling stones come from one small island off Scotland’s coast?

By Asuka Koda, CNN

(CNN) — Team USA won silver in mixed doubles curling on Tuesday after reaching the event’s final for the first time during the Winter Olympics. In the semifinal against Italy on Monday, American curler Cory Thiesse made the winning shot that knocked the Italian team’s curling stone out of its place.

The strictly regulated curling stones weigh between 38 and 44 pounds (17 and 20 kilograms) and can last decades. One company, Kays of Scotland, handcrafts most professional and all Olympic stones using granite from a single small uninhabited island off the coast of Scotland.

Granite from Ailsa Craig is exceptionally fine-grained; its minerals are arranged in such a way that tightly knits them together. This density makes the granite particularly resilient to collisions and allows it to be polished to a finish smooth enough to glide on ice. The unique mineral composition also gives the stones an intrinsic ability to curl along their trajectory.

“It’s not just about the ability to withstand chips and cracks. It also has to do with how it moves on the ice and how the stones bounce when they hit each other,” said Dr. Bob Gooday, a geological analyst at National Museums Scotland. “Professional curlers have used other kinds of stones, which slide perfectly well, but when they hit each other, they don’t bounce quite the same.”

Mark Callan, the chief ice technician of World Curling and the former sales and technical services director of Kays of Scotland, said the company produces 2,000 to 2,500 stones per year for 77 countries.

A special game takes a special stone

Curling was one of the first Winter Olympic sports, debuting as a medal event at the inaugural 1924 Winter Games in Chamonix, France. The sport originated in 16th century Scotland, where people slid rough stones on frozen lakes. Curlers take turns sliding granite stones across pebbled ice toward their “house,” a circular target marked on the surface.

The curling surface is specially prepared with frozen water droplets that create “pebbles” on the ice sheet, minimizing friction. The pebbled texture reduces surface area contact and prevents the heavy stone from sticking, allowing it to glide more easily on the ice.

An athlete creates a curved trajectory, known as the “curl,” which can be influenced by up to two sweepers who use brooms that melt the pebbled ice, reducing friction and allowing the stone to travel farther, sometimes slightly changing its direction. Curling is the only sport in which the projectile’s trajectory can be influenced after the athlete releases it.

A curling stone must be heavy enough to stay on its intended trajectory, withstand collision and glide on ice just enough so that it can still be influenced by sweeping. Most granites are not fit for this challenge.

“Most granites have very similar chemistry, and they tend to form lots of relatively large quartz and feldspar crystals. That texture obviously isn’t particularly good for curling stones,” Gooday said. Coarse stones with large crystals chip and fracture easily when knocked together.

However, granite from the island Ailsa Crag, which is off the west coast of Scotland, is around 60 million years old, Gooday explained. “This is the time that the North Atlantic is opening up, and Europe and North America are separating, so there’s formation of lots and lots of magma,” he said. The Ailsa Craig granite was formed from the cooling and solidifying of this magma.

Ailsa Craig granite has “slightly strange, unusual chemistry,” Gooday said. Compared with granites worldwide, the rock in Ailsa Craig is extremely low in aluminum. This geological anomaly led to the formation of rare minerals high in sodium and iron — such as arfvedsonite, aegirine and aenigmatite — that typically do not occur in granite.

For reasons scientists still do not fully understand, Gooday added, “the unusual mineralogy and the tight, fine structure of the granite itself” make Ailsa Craig granite resistant to chipping and ideal for curling stones.

The composition of curling stones

Three types of granite are found on Ailsa Craig: common green, blue hone and red hone. Gooday explained that red hone is blue hone that has been stained by iron materials. Common green and blue hone are used to make curling stones, and geochemically, the two types of granite are the same.

The body of the curling stone is made with common green. This includes the “striking band,” where stones collide. Common green granite contains black spots where rare minerals clump together, which gives the stone its “springy nature. These rocks sort of bounce off each other when they hit,” Gooday explained.

Blue hone is used to create the concave bottom of the curling stone, which forms a narrow “running band” that allows the stone to glide. This type of granite has low porosity and permeability, meaning that water does not penetrate the stone as easily, keeping it durable and consistent over time, explained Dr. Derek Leung, an assistant professor of mineralogy at the University of Regina in Saskatchewan and a curler at the Caledonian Curling Club.

This quality of blue hone is one reason why alternatives to Ailsa Craig granite do not compare. “We’ve tried to use other materials for the running surface,” Leung said. “One idea is that rocks are natural. They’re imperfect. So, what if we use something synthetic?” He added that members of the curling community tried using ceramics to build the running band.

However, Leung explained that ceramics cannot retain their texture over time. The roughness of a curling stone is what allows the rocks to curl. As a ceramic stone loses its texture over the course of a game, it becomes inconsistent and unable to curl.

The curling mechanism of a stone comes from the intrinsic characteristic of the granite, “the mineral grains of a certain size that are grown within rock,” according to Leung.

Ailsa Craig granite is so well-suited for curling that the Olympics have never used any other kind of granite in formal competition. “There is consensus that Ailsa Craig stones are the best,” said Dr. Matthew McDowell, a lecturer in sports policy, management and international development at Scotland’s University of Edinburgh.

The Grand Caledonian Curling Club, later renamed the Royal Caledonian Curling Club, has required the island’s granite for curling stones since its founding in 1838, he explained. The club serves as the sport’s governing body in Scotland and standardized the rules of play.

“Thus, when the Winter Olympics first took place in 1924 in Chamonix, France, the RCCC made the rules for the competition — and that included using Ailsa Craig granite for the stones,” he added.

McDowell said that, historically, there has never been a push to seek alternatives to Ailsa Craig granite.

“We know they’ve worked since the 1800s, so why change that?” Leung added.

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