Every four years, one winter sport in particular manages to capture the international zeitgeist for the briefest of moments before disappearing into obscurity until the next Olympics. Nestled in between events that require athletes to hurl themselves from the tops of mountains and launch themselves into the air with blades attached to their feet, curling functions as a mental reprieve for those of us who are more athletically challenged.

There’s something wonderfully ordinary about men and women of varying levels of fitness playing glorified shuffleboard on ice. However, as comparatively undemanding as curling is on the body, the same cannot be said for the curling stones themselves.

It turns out there are very few types of rock in existence that can withstand the stress of gliding along melting ice and smashing into more rock. Most granite is too quartz-rich for it to withstand the impact under curling conditions, which is what makes the granite found on a tiny deserted island off the Scottish coast so special.

Ailsa Craig is 10 miles west of mainland Scotland – and while it was once inhabited by Catholics who sought refuge during the Scottish Reformation, it is now an unpopulated safe haven for various birds and serves as the world’s only source for the rare granite that is used to make Olympic curling stones.

Also known as “Paddy’s milestone,” the island is actually a volcanic plug – a literal rock formed over an extinct volcano. And it turns out that the unique makeup of the rock left behind is uniquely suited for making curling stones.

There are three subgroups of the water-resistant microgranite that can can be mined from the island: Ailsa Craig Common Green Granite, Ailsa Craig Blue Hone Granite and Ailsa Craig Red Hone Granite.

Kays of Scotland has been making curling stones since 1851 and has the exclusive rights to the Ailsa Craig granite, which is why it has provided the stones for every single competition at the Olympic Winter Games. About every 10 years, Kays extracts several thousand tons of the distinctive blue and green varieties. The blue hone, whose tight molecular structure makes it impervious to water and melting ice, is used for the insert and running band of the stone, while the green granite makes up the body.

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