Sports and leisure

Isla Rennick: Thank you. Most people's picture of curling is of two players sweeping furiously in front of a sliding stone, and it's the sweeping that tends to get laughed at on television. I'm a geologist, so I'm afraid I'm going to ask you to ignore the brooms and look at the stone. Isla Rennick: A competition stone weighs roughly twenty kilos, but surprisingly little of it ever touches the ice. Turn one over and you'll see that the base is slightly hollowed, so the stone rests on a narrow ring only a few millimetres wide, called the running band. Around the middle of the stone's side there's a second band, the striking band, which takes the impact when one stone knocks another out of the way. The two have completely different jobs, and on the best stones they're made from different granites. Isla Rennick: Why granite at all? The usual answer is hardness, and the striking band certainly has to survive thousands of collisions. But plenty of granites are hard enough. What rules out almost all of them is that they absorb water. The surface of the ice is always slightly wet, and if water gets into the tiny gaps between the crystals of the running band and then freezes, it expands and flakes off little chips of stone. A stone that's shedding chips curls unpredictably, and a player can't trust it. So for the running band we need a granite with extraordinarily few gaps, and there are very few places on earth where that's found. Isla Rennick: The source I work with is a small island, which I won't name, because its owners would quite like to keep visitors away. It's a breeding ground for seabirds, and that shapes everything about how the stone is collected. People imagine blasting and heavy machinery. In fact nobody is permitted to cut into the rock face at all. The company may only take blocks that have already fallen from the cliffs, and only during a few weeks of late autumn, once the young birds have left. The last collection was nine years ago, and it produced enough granite to last until the 2040s, provided nobody drops a block over the side of the boat, which has happened. Isla Rennick: When a block reaches the workshop, the first question is whether it's sound all the way through. The old stonemasons used to strike each block with a hammer and listen: a clear ring meant solid rock, a dull note meant a hidden crack. I was rather sorry to find that the workshop now uses ultrasound, until I learned that the men who listened were wrong about one time in five. Isla Rennick: One last thing surprised me more than anything else. Stones are sold in sets of sixteen, and all sixteen have to behave identically, so a set is always cut from a single block. That means that if a club loses a stone, cracked or stolen, it can't simply order a replacement. The new stone would come from different rock and run very slightly differently, and good players can feel it. Most clubs in that position end up buying a whole new set.