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