Curling Stone
The curling stone is circular and weighs between 17-20kg with a handle and bolt attached. The stone has a maximum circumference of 910 mm. A stone must be a minimum of 110 mm in height.
The handle is attached by a bolt that runs vertically through a hole in the centre of the stone. The handle allows the rock to be gripped and rotated upon release. The handles are coloured to tell which rocks belonging to each team. Two popular colours in major tournaments are red and yellow.
The top and bottom of a curling stone are concave. The surface in contact with the ice, the running surface, is a circle 6.3-13 mm thick. On properly prepared ice, the rock's path will bend (curl) in the direction the front edge of the rock is turning, especially toward the end of its motion.
The Scots, in particular, believe that the best-quality curling stones are made from a specific type of granite called "ailsite," found on the Ailsa Craig, an island off the Ayrshire coast. Ailsite has very low water absorption, which prevents the action of freezing and melting water from eroding the stone. However, the island is now a wildlife reserve and is no longer used for quarrying. Many curling clubs use a lower-grade stone. There are also stones that use a disc with a running surface of Ailsite attached below another type of granite. Informal curling clubs with limited resources may make cylindrical "curling stones" out of concrete-filled cans.