Riebeckite Microgranite in Curling Stones

Written by Connor Clinton, Science Communication Student at the University of Sheffield

10 miles off the coast of the Firth of Clyde lies one of Scotland’s most striking landmarks. The small island of Ailsa Craig rises 338 metres above the Atlantic Ocean and is composed almost entirely of riebeckite microgranite.

The island was formed 60 million years ago when magma rose through the Earth's crust but cooled and solidified before reaching the surface, forming a dense, resistant plug of rock. Erosion wore away the softer rock around it, leaving Ailsa Craig standing alone in the sea. The mineralogy of microgranite on the island makes it geologically distinctive, distinguished by a black spotted appearance resulting from black clots of ferromagnesian minerals in the rock. One of the ferromagnesian minerals present is riebeckite, explaining the name of the stone.

The spotted, fine-grained texture of the rock is not only distinctive but has a unique value in the sport of curling. Quarrying on the island has supplied curling stones for the sport globally since the early nineteenth century, including all 132 used in the 2026 Milano Cortina Winter Olympics.

A curling stone is made from two varieties of Ailsa Craig microgranite, each suited to a different part of the stone. The body of the stone is carved from Common Green Ailsa, chosen for its resistance to heat transfer and its ability to withstand repeated impacts without splintering. The running band (the smooth ring at the base of the stone that makes contact with the ice) is fitted separately using Ailsa Craig Blue Hone, a variety with exceptionally low water absorption that allows the stone to glide smoothly across the ice without fracturing.