Originally published in January 2015 for WindyCityCurling.com.

Let’s talk about rocks.

OK, not those rocks. Stones, curling stones.

Here’s the wikipedia entry for curling stones.

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 and 20 kg) a maximum circumference of 36 inches (910 mm) and a minimum height of 4.5 inches (110 mm).[15] 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, Canada sometime after 1870, and was subsequently adopted by Scottish stone manufacturer Andrew Kay.[9]

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.

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.[19] 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. Kays of Scotland has been making curling stones since 1851 and has the exclusive rights to the Ailsa Craig granite, granted by the Marquess of Ailsa, whose family has owned the island since 1560. The last harvest of Ailsa Craig granite by Kays took place in 2013, after a hiatus of 11 years; 2,000 tons were harvested, sufficient to fill anticipated orders through at least 2020. Kays has been the exclusive manufacturer of curling stones for the Olympics since the 2006 Winter Olympics.[20]

Trefor granite comes from the Yr Eifl or Trefor Granite Quarry in the village of Trefor on the north coast of the Llŷn Peninsula in Gwynedd, Wales and has produced granite since 1850. Trefor granite comes in shades of pink, blue and grey.[21] The quarry supplies curling stone granite exclusively to the Canadian, Canada Curling Stone Co., which has been producing stones since 1992 and supplied the stones for the 2002 Winter Olympics.

A handle is attached by a bolt running vertically through a hole in the centre of the stone. The handle allows the stone to be gripped and rotated upon release; on properly prepared ice the rotation will bend (curl) the path of the stone in the direction in which the front edge of the stone is turning, especially as the stone slows. Handles are coloured to identify each team; two popular colours in major tournaments being red and yellow. In competition, an electronic handle known as the eye on the hog may be fitted to detect hog line violations, the game’s most frequent cause of controversy. This electronically detects whether the thrower’s hand is in contact with the handle as it passes the hog line and indicates a violation by lights at the base of the handle. The eye on the hog eliminates human error and the need for hog line officials. It is mandatory in high-level national and international competition, but its cost, around US$650 each, currently puts it beyond the reach of most club curling.

So, now that you know about curling stones.  Let’s see how they are made!