<P> A key part of the preparation of the playing surface is the spraying of water droplets onto the ice, which form pebble on freezing . The pebbled ice surface resembles an orange peel, and the stone moves on top of the pebbled ice . As the stone moves over the pebble, any rotation of the stone causes it to curl to the inside or outside . The amount of curl (commonly referred to as the feet of curl) can change during a game as the pebble wears; the ice maker must monitor this and be prepared to scrape and re-pebble the surface prior to each game . </P> <P> The curling stone (also sometimes called a rock in North America) is made of granite and is specified by the World Curling Federation, which requires a weight between 38 and 44 pounds (17.24 and 19.96 kg), a maximum circumference of 36 inches (914.4 mm) and a minimum height of 4.5 inches (114.3 mm). The only part of the stone in contact with the ice is the running surface, a narrow, flat annulus or ring, 0.25 to 0.50 inches (6.4 to 12.7 mm) wide and about 5 inches (130 mm) in diameter; the sides of the stone bulge convex down to the ring and the inside of the ring is hollowed concave to clear the ice . This concave bottom was first proposed by J.S. Russell of Toronto, Ontario, Canada sometime after 1870, and was subsequently adopted by Scottish stone manufacturer Andrew Kay . </P> <P> The granite for the stones comes from two sources: Ailsa Craig, an island off the Ayrshire coast of Scotland, and the Trefor Granite Quarry in Wales . </P> <P> Ailsa Craig is the traditional source and produces two types of granite, Blue Hone and Ailsa Craig Common Green . Blue Hone has very low water absorption, which prevents the action of repeatedly freezing water from eroding the stone . Ailsa Craig Common Green is a lesser quality granite than Blue Hone . In the past, most curling stones were made from Blue Hone but the island is now a wildlife reserve and the quarry is restricted by environmental conditions that exclude blasting . </P>

Where does granite for curling stones come from