When the Winter Olympics commence this month in Sochi, Russia, there should be no shortage of heart-stopping action. Alpine skiers will carve downhill turns at 80 miles per hour. Hockey players will battle one another for the puck. Snowboarders will twist and flip multiple times in a single jump. And then there is curling, in which a more sedate bunch will push a 44-pound rock down a long sheet of ice and then sweep the ice with brooms to “curl” that stone toward a target.

Although curling is an Olympic newcomer, having been officially included only since 1998, the stones have a long, rich history of their own. “Every single Olympic curling stone comes from this little island off the coast of Scotland, called Ailsa Craig,” says Erika Brown, skipper of the U.S. women's team. “And no other stone curls like an Ailsa Craig stone.”

The 220-acre island, about 10 miles from mainland Scotland, is the source of two varieties of granite used in elite curling stones. Blue hone granite makes up the layer that glides down the ice, and common green granite makes up the middle layer, or striking band. “The layer of rock that runs along the ice doesn't chip or absorb water, but most important, it's very predictable on ice—you know what your shots are going to do,” Brown says. “And the middle layer doesn't break when the stones collide.”

The stones' performance traces back to the island's formation about 60 million years ago. Ailsa Craig is a volcanic intrusion—a mass of magma that forced its way up between existing formations—explains John Faithfull, a geologist at the University of Glasgow. The magma then cooled relatively quickly to form granite, and the surrounding rock eroded away, “leaving just the very resistant hard mass of Ailsa Craig poking up out of the water,” Faithfull says.

As the volcanic rock crystallized, it developed a strong, uniform surface. “When magma cools quickly, it creates very small crystals. These ones interlocked, and chemical bonds developed between them,” says Martin Gillespie, a geologist at the British Geological Survey. “It also doesn't seem to have any microcracks,” he says of the granite.

The granite's unique qualities make Ailsa Craig stones the “gold standard,” Brown says. “For us curlers, the island is a mystical place.”